Mac OS X only: Open-source application Books is a personal database of all the books you own, read, borrowed, or lent out. With hooks into Amazon and other book information services, you can type the title of a book into Books, and automatically suck in the author, ISBN, cover art, summary, publisher, and other metadata right into the app. Enter multiple reviews and notes for each book, and you can even attach files to a book (like, say, a sample chapter you downloaded). Keep track of who you lent a book to and when as well, and make smart lists of books based on criteria—like all books whose keyword contains the word "business," or all books by a particular author. Take a look at some screenies of Books in action.
You can list your books in "gallery" mode to see cover art:
Here's an individual book record:
While Delicious Library is probably the best pay-for version of a Mac media library tracker, Books is fantastic, free, books-only solution for Mac users. Books is a free download for Mac OS X only.