Google Book Search is a service that lets you search more than 500 million pages of scanned books and the number grows every day. But why would you need that when you could go to a library? Well, sometimes it's hard to discover books only from the title and description and once you discovered them it's hard to actually find them in libraries and book stores.
1. You have a quote from a book, but you don't know where it's from.
2. You have a quote from a book, and you want to find the context. Maybe you want to read the poem that contains the famous "eternal sunshine of the spotless mind".
3. Maybe you have a book and you want to read again a certain fragment. But it's hard to find it, even though you remember some keywords.
4. Search your bookshelves. You can't restrict the search to your books, but if you enter something unique (name of a character), you'll discover the book.
5. Search within books published in a certain year, to see different perspectives on a subject.
6. Find references from other books to a certain book.
7. Search for an affirmation or the prefix of an affirmation ("Paris is the only city...").
8. Find contexts for rare or difficult words (ineluctable).
9. Read out-of-copyright books online or download them as PDF files.
10. Detect plagiarism, as this article shows.
Related:
Google's Digital Library of Alexandria
Download public domain books