Google Book Search gets you inside plenty of books, but Project Gutenberg still offers more classics.
With Google Book Search beta, you can either download PDF files of books that aren't under copyright lock or view book excerpts in PDF. An Advanced Book Search lets you narrow down queries by author, title, date, publisher, and ISBN number. We liked that the program's clean interface highlights every instance of your search term in the PDF. And, of course, you can buy many of the books through Google's interface as well.
While Google is making good progress with its book project, the early innovator in this area is Project Gutenberg. This Web site offers some 19,000 books for download in HTML, plain text, or as files suited for the Plucker e-book reader for PDAs. We found Gutenberg easier for locating popular classics, such as Shakespeare, Mark Twain, and Jane Austen, for example. Yet Google is still building its library, and includes more contemporary works.