diff --git a/migrations/versions/5584e4fd820e_add_unique_constraint_to_isbn_field.py b/migrations/versions/5584e4fd820e_add_unique_constraint_to_isbn_field.py
new file mode 100644
index 0000000..e7ed915
--- /dev/null
+++ b/migrations/versions/5584e4fd820e_add_unique_constraint_to_isbn_field.py
@@ -0,0 +1,64 @@
+"""Add unique constraint to ISBN field
+
+Revision ID: 5584e4fd820e
+Revises: 0c155d83c55b
+Create Date: 2026-03-31 13:41:16.356631
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = '5584e4fd820e'
+down_revision = '0c155d83c55b'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    
+    # First, convert empty string ISBNs to a temporary placeholder
+    # This is needed because we can't set NULL while column is NOT NULL
+    op.execute("UPDATE book SET isbn = '__EMPTY_ISBN__' WHERE isbn = ''")
+    
+    # Remove duplicate ISBNs, keeping only the book with the lowest ID
+    op.execute("""
+        DELETE FROM book 
+        WHERE isbn != '__EMPTY_ISBN__' 
+        AND isbn != '' 
+        AND id NOT IN (
+            SELECT MIN(id) 
+            FROM book 
+            WHERE isbn != '__EMPTY_ISBN__' 
+            AND isbn != '' 
+            GROUP BY isbn
+        )
+    """)
+    
+    with op.batch_alter_table('book', schema=None) as batch_op:
+        # Make the column nullable
+        batch_op.alter_column('isbn',
+               existing_type=sa.VARCHAR(length=20),
+               nullable=True)
+    
+    # Now convert the placeholder values to NULL
+    op.execute("UPDATE book SET isbn = NULL WHERE isbn = '__EMPTY_ISBN__'")
+    
+    with op.batch_alter_table('book', schema=None) as batch_op:
+        # Add the unique constraint
+        batch_op.create_unique_constraint('uq_book_isbn', ['isbn'])
+
+    # ### end Alembic commands ###
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    with op.batch_alter_table('book', schema=None) as batch_op:
+        batch_op.drop_constraint('uq_book_isbn', type_='unique')
+        batch_op.alter_column('isbn',
+               existing_type=sa.VARCHAR(length=20),
+               nullable=False)
+
+    # ### end Alembic commands ###
diff --git a/src/hxbooks/cli.py b/src/hxbooks/cli.py
index 2cd8c20..2c456b7 100644
--- a/src/hxbooks/cli.py
+++ b/src/hxbooks/cli.py
@@ -16,6 +16,7 @@ from flask import Flask
 from . import library
 from .app import create_app
 from .db import db
+from .library import DuplicateISBNError
 from .models import Author, Book, Genre, Reading, User, Wishlist
 
 logger = logging.getLogger(__name__)
@@ -129,6 +130,9 @@ def add_book(
                 cover_image_url=cover_url,
             )
             click.echo(f"Added book: {book.title} (ID: {book.id})")
+        except DuplicateISBNError as e:
+            click.echo(f"Error: {e}", err=True)
+            sys.exit(1)
         except Exception as e:
             logger.error(f"Error adding book '{title}': {e}", exc_info=True)
             click.echo(f"Error adding book: {e}", err=True)
@@ -504,6 +508,9 @@ def import_book(
             click.echo(
                 f"Imported book: {book.title} by {', '.join(a.name.title() for a in book.authors)} (ID: {book.id})"
             )
+        except DuplicateISBNError as e:
+            click.echo(f"Error: {e}", err=True)
+            sys.exit(1)
         except Exception as e:
             click.echo(f"Error importing book: {e}", err=True)
             sys.exit(1)
diff --git a/src/hxbooks/db.py b/src/hxbooks/db.py
index f72f9f8..b582587 100644
--- a/src/hxbooks/db.py
+++ b/src/hxbooks/db.py
@@ -6,7 +6,7 @@ from sqlalchemy.orm import DeclarativeBase
 class Base(DeclarativeBase): ...
 
 
-db = SQLAlchemy(model_class=Base)
+db = SQLAlchemy(model_class=Base, session_options={"autoflush": False})
 
 
 def init_app(app: Flask) -> None:
diff --git a/src/hxbooks/library.py b/src/hxbooks/library.py
index 5389708..571d6d9 100644
--- a/src/hxbooks/library.py
+++ b/src/hxbooks/library.py
@@ -16,6 +16,7 @@ import requests
 from flask import current_app
 from PIL import Image
 from sqlalchemy import ColumnElement, Select, and_, func, or_, select
+from sqlalchemy.exc import IntegrityError
 from sqlalchemy.orm import InstrumentedAttribute, joinedload
 
 from hxbooks.search import IsOperatorValue, QueryParser, SortDirection, ValueT
@@ -29,6 +30,20 @@ from .search import ComparisonOperator, Field, FieldFilter
 logger = logging.getLogger(__name__)
 
 
+class DuplicateISBNError(ValueError):
+    """Raised when attempting to create or update a book with a duplicate ISBN."""
+
+    def __init__(
+        self, isbn: str, existing_book_id: int, existing_book_title: str
+    ) -> None:
+        self.isbn = isbn
+        self.existing_book_id = existing_book_id
+        self.existing_book_title = existing_book_title
+        super().__init__(
+            f"ISBN '{isbn}' is already used by book '{existing_book_title}' (ID: {existing_book_id})"
+        )
+
+
 def create_book(
     title: str,
     owner_id: int | None = None,
@@ -52,7 +67,7 @@ def create_book(
     book = Book(
         title=title,
         owner_id=owner_id,
-        isbn=isbn or "",
+        isbn=isbn,
         publisher=publisher or "",
         edition=edition or "",
         description=description or "",
@@ -79,7 +94,20 @@ def create_book(
             genre = _get_or_create_genre(genre_name)
             book.genres.append(genre)
 
-    db.session.commit()
+    try:
+        db.session.commit()
+    except IntegrityError as e:
+        db.session.rollback()
+        if "book.isbn" in str(e.orig).lower():
+            # Find the existing book with this ISBN
+            existing_book = db.session.execute(
+                select(Book).filter(Book.isbn == isbn)
+            ).scalar_one_or_none()
+            if existing_book:
+                raise DuplicateISBNError(
+                    isbn or "", existing_book.id, existing_book.title
+                ) from e
+        raise
 
     # Handle cover image if provided
     if cover_image_url:
@@ -137,7 +165,6 @@ def update_book(
         assert title is not None, "Title is required when set_all_fields is True"
         book.title = title
     if isbn is not None or set_all_fields:
-        assert isbn is not None, "ISBN is required when set_all_fields is True"
         book.isbn = isbn
     if publisher is not None or set_all_fields:
         assert publisher is not None, (
@@ -208,7 +235,21 @@ def update_book(
         else:
             remove_book_cover(book)
 
-    db.session.commit()
+    try:
+        db.session.commit()
+    except IntegrityError as e:
+        db.session.rollback()
+        if "book.isbn" in str(e.orig).lower():
+            # Find the existing book with this ISBN (excluding current book)
+            existing_book = db.session.execute(
+                select(Book).filter(Book.isbn == isbn, Book.id != book.id)
+            ).scalar_one_or_none()
+            if existing_book:
+                raise DuplicateISBNError(
+                    isbn or "", existing_book.id, existing_book.title
+                ) from e
+        raise
+
     return book
 
 
diff --git a/src/hxbooks/main.py b/src/hxbooks/main.py
index dc9f7a4..2ac9c16 100644
--- a/src/hxbooks/main.py
+++ b/src/hxbooks/main.py
@@ -9,7 +9,7 @@ import os
 import tempfile
 from datetime import date
 from pathlib import Path
-from typing import Annotated, Any, Literal
+from typing import Annotated, Any
 
 from flask import (
     Blueprint,
@@ -41,6 +41,7 @@ from hxbooks.search import IsOperatorValue, SortDirection
 
 from . import library
 from .db import db
+from .library import DuplicateISBNError
 
 bp = Blueprint("main", __name__)
 
@@ -51,7 +52,7 @@ logger = logging.getLogger(__name__)
 # Pydantic validation models
 StrOrNone = Annotated[str | None, BeforeValidator(lambda v: v.strip() or None)]
 StripStr = Annotated[str, StringConstraints(strip_whitespace=True)]
-ISBNOrEmpty = Annotated[ISBN | Literal[""], BeforeValidator(lambda v: v.strip() or "")]
+ISBNOrNone = Annotated[ISBN | None, BeforeValidator(lambda v: v.strip() or None)]
 TextareaList = Annotated[
     list[str],
     BeforeValidator(
@@ -70,7 +71,7 @@ UrlOrNone = Annotated[AnyHttpUrl | None, BeforeValidator(lambda v: v.strip() or
 class BookFormData(BaseModel):
     title: StripStr = Field(min_length=1)
     owner: StrOrNone = None
-    isbn: ISBNOrEmpty = ""
+    isbn: ISBNOrNone = None
     authors: TextareaList = Field(default_factory=list)
     genres: TextareaList = Field(default_factory=list)
     first_published: IntOrNone = Field(default=None, le=2030)
@@ -283,7 +284,7 @@ def create_book() -> ResponseReturnValue:
                 owner_id=owner_id,
                 authors=form_data.authors,
                 genres=form_data.genres,
-                isbn=str(form_data.isbn),
+                isbn=form_data.isbn,
                 publisher=form_data.publisher,
                 edition=form_data.edition,
                 description=form_data.description,
@@ -300,6 +301,10 @@ def create_book() -> ResponseReturnValue:
             flash(f"Book '{form_data.title}' created successfully!", "success")
             return redirect(url_for("main.book_detail", book_id=book.id))
 
+        except DuplicateISBNError as e:
+            flash(f"Error: {e}", "error")
+            return render_template("book/create.html.j2", form_data=request.form)
+
         except ValidationError as e:
             _flash_validation_errors(e)
 
@@ -357,6 +362,9 @@ def update_book(book_id: int) -> ResponseReturnValue:
 
         flash("Book updated successfully!", "success")
 
+    except DuplicateISBNError as e:
+        flash(f"Error: {e}", "error")
+
     except ValidationError as e:
         # Format validation errors for display
         _flash_validation_errors(e)
diff --git a/src/hxbooks/models.py b/src/hxbooks/models.py
index 940e870..1a27f0f 100644
--- a/src/hxbooks/models.py
+++ b/src/hxbooks/models.py
@@ -56,7 +56,9 @@ class Book(db.Model):  # ty:ignore[unsupported-base]
     first_published: Mapped[int | None] = mapped_column(default=None)
     edition: Mapped[str] = mapped_column(String(200), default="")
     publisher: Mapped[str] = mapped_column(String(200), default="")
-    isbn: Mapped[str] = mapped_column(String(20), default="")
+    isbn: Mapped[str | None] = mapped_column(
+        String(20), unique=True, nullable=True, default=None
+    )
     notes: Mapped[str] = mapped_column(default="")
     added_date: Mapped[datetime] = mapped_column(default=datetime.now)
     bought_date: Mapped[date | None] = mapped_column(default=None)
diff --git a/src/hxbooks/templates/components/book_form.html.j2 b/src/hxbooks/templates/components/book_form.html.j2
index 6c827fb..a4fb555 100644
--- a/src/hxbooks/templates/components/book_form.html.j2
+++ b/src/hxbooks/templates/components/book_form.html.j2
@@ -12,7 +12,7 @@
   </div>
   <div class="col-md-3">
     <label for="isbn" class="form-label">ISBN</label>
-    <input type="text" class="form-control" id="isbn" name="isbn" value="{{ book.isbn if book else '' }}">
+    <input type="text" class="form-control" id="isbn" name="isbn" value="{{ book.isbn if book and book.isbn else '' }}">
   </div>
 </div>
 
@@ -73,7 +73,7 @@
       <div class="col-md-6">
         <label for="cover_image_url" class="form-label">Replace with URL</label>
         <input type="url" class="form-control" id="cover_image_url" name="cover_image_url" value=""
-          placeholder="https://example.com/cover.jpg or file:///path/to/image.jpg">
+          placeholder="https://example.com/cover.jpg">
       </div>
       <div class="col-md-6">
         <label for="cover_image_file" class="form-label">Replace with Upload</label>