The FAA recently published a new edition of the Glider Flying Handbook. It looks like some nice changes were made to clarify a few points the old version didn’t do quite so well. They also did some work reorganizing it a bit and I think that made for some improvement too.
If you have something comfortable for reading PDFs you can save a few bucks by going to http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/glider_handbook/ and downloading it. If you’re a glider pilot, or just curious, it’s a worthwhile read.