I had previously mentioned a resource for free downloadable chess books (check the posts for April 2007), and I have just come across another one. A website called Bookyards has a collection of chess books (and a whole lot of other books) for downloading in pdf format. Simply visit the site and do a search on "chess" to see what comes up.
While the majority of books seem old enough to be out of copyright (and in the public domain) there seems to be a number of more recent books such as "The Art Of Attack In Chess" that I wonder about the copyright status of. The website has a legal disclaimer stating that all books are in the Public Domain but if you're not sure of the legality, I'll leave the decision up to you.
The only other thing to note is before downloading a book you have to wait about 15 seconds while a short video is played. What the video is for, and what videos are shown (seems to change each time) are at this stage unknown to me. (So viewer beware!)