My coworker, Kevin Ring, has joined me as a coauthor to write a few of the final, but core, chapters. Kevin has served as a reviewer for many of the chapters I already wrote, has contributed quite a bit to the book's example code (including the message queue and administering the continuous integration server). He even contributed some book content to the section on implementing a message queue. At this point, he is practically a coauthor anyway.
He will be writing about terrain LOD and imagery tiling, while I finish up chapters on renderer design and precision. We have plenty to do by the end of the year but I think we are in good shape, and I am not one to wait until the last minute to panic.