Create an online book swap account with websites like bookart.com, bookins.com and paperbackswap.com. Once you have an account with these sites, you can search for books and offer to swap a book you have with a fellow book swapper who has a book that you'd like.
Download free e-books or pdf books at websites like getfreebooks.com, worldlibrary.net and manybooks.net. You'll need to create an account with these websites, but once you have signed up you can search for, download and read books for free.
Contact your local library or used book stores to find out if they are trying to get rid of any books. Sometimes libraries and used book stores have more stock than they can handle, and they may just give you free books if you ask.
Contact the publishing companies of the books you're interested in receiving. In certain cases -- such as when a book is no longer being sold or a publisher wants to get rid of their stock -- they may decide to give you a book for free. Publishing companies may also have online contests that allow you to receive free books. Check their websites for more details.