Amazon offers daily deals for kindle, including free e-books. This is a great opportunity even if you don't have a kindle. There is a kindle mobile app available for both Apple and Android products, or you can read right on your computer. Here are the books that are available for free today, March 27.
Disclaimer: I haven't read any of these books, so I don't know if they are useful. Please comment if you've read them and tell me what you think!
Healing Crystals: A Guide to Working With Obsidian by Brenda Hunt
Money in the Tarot by Carole Sommerville
Magickal Aids in Wicca by Lady Moonraven
A Beginner's Manual of Magic by Benjamin Miro