Books
Google Books
Some books are offered completely free, while other publishers provide previews. Sometimes, if you're lucky, the section of the book you need will be contained within the preview.
Gutenburg.org
If you are looking for a classic, you may be able to find it for free here. Gutenberg.org hosts thousands of free public domain ebooks. You'll be able to find classics such as The Origin of Species here.
Merck Veterinary Manual
Free online version of this comprehensive book.
Paperbackswap.com
On paperbackswap.com you can exchange books that you no longer want for new books. When you send someone a book, you receive a one credit good for requesting one book from another user. All you pay is the shipping on the books you send. You might not be able to find exactly what you're looking for, but sometimes you'll be surprised. Currently you receice two free credits just for listing your first ten books.