Sign up for online book swap. There are a number of different websites, like paperbackswap.com, bookins.com and the bookcart.com, that allow you to obtain free books -- including building books --once you create an account and offer books that you are willing to swap with others.
Contact your local library and ask a librarian on duty if the library has the ability to mail books to community members. To qualify for this type of service you will need a valid library card, and you may have to pay for the mailing charge.
Contact the publisher of a building book that you are interested in acquiring. If the book is no longer being sold or the publisher simply wants to get rid of the stock, it may agree to send you the book free of charge. This option is the least likely of the three, however, so only try this option as a last resort.