As mentioned previously, this book focuses on open-source UNIX operating systems, mainly because of transparency reasons and the availability of compilers and tools. I do not include NetBSD in this discussion because it is tailored for portability and does not offer anything remarkable beyond OpenBSD or FreeBSD.
The most dominant commercial operating systems right now are Sun Solaris, Hewlett-Packard HP-UX, and IBM AIX. BSDI is Riverstone's commercial branch of BSD, facilitating Nexthop's GateD code as well as their own extensions. Almost all the big players have additional Linux-centric lines of business.