KGPJ is the abbreviation for my earlier book, Killer Game Programming in Java.
KGPJ is about 2D, 3D, and network games programming, but Pro Java 6 3D Game Development (PJ3D) concentrates solely on 3D programming.
KGPJ has sixteen chapters on Java 3D, while PJ3D has eight (roughly half the book), and also covers nonstandard input devices and JOGL. Of those eight chapters, four are on topics not covered in KGPJ, namely Java SE 6 integration, physics modeling, multitexturing, and mixed-mode rendering. The other four chapters overlap with KGPJ to some degree, but it means that PJ3D is completely self-contained; there's no need for you to have to read KGPJ before starting PJ3D. Also, all the example programs are entirely new.
KGPJ doesn't discuss the range of APIs covered in PJ3D, such as JOGL, JInput, JOAL, Odejava, and JMF.
KGPJ was published in May 2005 so focuses on J2SE 1.4 and Java 3D 1.3.1, while PJ3D utilizes Java SE 6 and Java 3D 1.5. It's due out in April 2007.
If you're still not sure, the best solution, at least the one most pleasing to me, is to buy both books. Thank you :).