Right now, I'm actually working on 3 books - sort of. Although Beginning OpenGL Game Programming shipped in March, Kevin and I are still obliged to put together a resource CD. BOGLGP was written so that it could be used as a text book, so the resource CD will contain slideshows, additional exercises, etc, for instructors. Needless to say, it's not the most exciting thing in the world to work on, which is why we're now 3 weeks late in delivering it.
The next book I'm due to finish (end of July) is OpenGL ES Game Development. I'm doing this with one of my coworkers. OpenGL ES is a subset of OpenGL developed for mobile devices, primarily cell phones. I've spent the past year working on a commecial software implementation of it (which will ship on hundreds of millions of devices), so I know a thing or two about it. Besides OpenGL ES, the book provides a lot of information for anyone doing mobile games, including information on the leading 3D mobile game engines, fixed point math, mobile audio, the business model, a case study of porting a PC game to mobile, and more. We were able to get quite a few leaders in the mobile gaming community to contribute to the book, so it should be quite good.
The third book is Advanced OpenGL Game Programming, which Kevin may or not be involved with. It'll pick up where BOGLGP left off, covering topics such as shaders (both OGLSL and asm shaders), VBOs, PBOs, special effects, occlusion queries, scene management, etc. The book's set to print around GDC next year.
But because of site related issues, I'm not getting a lot done on these books. Fortunately, we've got a number of people helping out (Oluseyi, Michalson, superpig, Sicrane, to name a few), so I should be able to stop spreading myself so thin, and actually get a few things done.