The two main sites for books on the open web are Google Books and Microsoft's rival version, Live Search Books. In general, Google Books has a larger selection, but Microsoft has a big advantage so far as new books are concerned. Some publishers allow Google Books readers access to up to 10% of a given book's content. This is very useful in many cases, but you can rarely access more than 5 or so pages in a row before running into pages that are withheld -- fair enough, they want you to buy the book.
Publishers working with Live Search Books, however, grant up to 20% access to a book's content (once you enroll and sign up -- free, just need an email account) and that 20% can be contiguous. It might well, for example, include a complete chapter in a book of essays. You cannot print effectively from these current books (out-of-copyright works can be dowloaded and printed in part or in whole as pdf files from either Google Books or Live Search Books), but, hey, you can read. In many cases the books are not in the library or out on loan, or just too new to be found elsewhere.