I came across this guys site a while ago. He has dozens of free downloadable ebooks, many by well known forex authors. I can not vouch for the legality of his site nor the safety of downloading from his site. I downloaded a few that were fine, but the end of the day you have to take that decision yourself. The site can be found here:
Free Forex Books, Download Best Forex Books, Forex E-Books
Categories he has listed;
Forex Books for Beginners
Books about Forex Market in General
Forex Books about Trading Psychology
Money Management Forex Books
Books about Forex Strategy
Books for Advanced Forex Traders