The Wings Go Club has a bunch of PDF Go books including a beginners guide, commented games, and some interesting guides to making the most out of handicap stones (the latter are also available in .doc format).
If you prefer physical books and are in Australia, J.W. & S. Hardy have a nice selection.
Image by Jim Kiraly used under CC licence. Jim is twelve ranks above me at present and is, at the time of writing, kicking the virtual snot out of me in a handicap match at DGS. I could've sure used those handicap books five moves earlier :)
ebooks