Voyager

IBM Redbooks

 * SG24-4630-00 OS/2 Warp (PowerPC Edition) A First Look
 * SG24-4639-00 OS/2 Warp (PowerPC Edition) Graphical Subsystem

We do have the first one digitaly, but not the second one. In case anyone finds that, let us know.

Darwin Documentation
There is plenty of documentation available about Darwin, a good start is Kernel Programming, which gives a nice overview about the design of the kernel.