textbooktorrents doesn't have any downloads on it anymore fyi.
I've used
www.bigwords.com for the past year and love it. It's basically a huge book search engine that looks through tons of cheap book websites and gives you the absolutely cheapest deals possible. I've probably saved about $800 dollars over 3 semesters on textbooks. They give you the option to rent textbooks from sites like chegg.com, or get international paperback editions (highly recommended if you dont care about color pages) for really cheap. If you don't like any of their offers you can click "Ignore" and they'll give you the next best. They also find coupons for you to make things cheaper. It's great.
edit: Finding ebook downloads is really difficult. I don't know of any reliable site for book downloads, other than maybe
http://www.ebooksbay.org/, which isn't very good. I'd recommend just googling "Book name author rapidshare" or "Book name author pdf" and look through the first 8-10 pages of results. Unless your book is some immensely popular textbook, you will probably not find anything. Probably common sense, but older editions of textbooks are 3x easier to find and download.
edit2: Note about international editions: If you choose to take this route, understand that your textbook may take 2+ weeks to get here. You also can't return it at your college bookstore, but they ARE ~70-90% off the retail price.