That's an incredible book if you want to understand the core of the various BSD kernels (obviously it's best for FreeBSD). It's the latest descendant of "The Design and Implementation of the 4.3BSD UNIX Operating System" which he wrote in 1989, a series of books that are justly famous. McKusick has been involved with the BSDs since he was Bill Joy's officemate and I'm pretty sure wrote the first fsck and most of the first usable filesystem for BSD. The filesystems section of the 4.4BSD version of the book was a fascinating read. He's also the guy responsible for the little red BSD daemon.

McKusick and his partner Eric Allman (sendmail & syslog) are the kind of older open source developers who're famous to Unix people and more Unix-inclined Linux people, but aren't as well known now as Stallman or de Raadt, say. Even now there's a high chance than any human being using the net has had their email, IM, or web traffic pass through software written by either of them.