I never read the original USAH so I have to ask: does this cover specific programs present on specific systems, or is it more of a philosophical guide to being a sysadmin?
My observation is that the recent direction of Fedora (which has been followed relatively closely by Ubuntu, RHEL, SUSE, etc.) has been diverging from traditional UNIX, as far as system administration tasks go. SysVinit is being replaced with systemd. You've got plymouth and kernel mode-setting for boot, instead of a text-mode console. PolicyKit runs a daemon that manages permissions and controls what apps can do. /dev/dsp has gone the way of the dinosaur, with another daemon -- pulseaudio -- taking its place. Then there's virtualization, which everybody does differently.
The evolution of the userspace plumbing layer of Linux over the past 10 years will change the way sysadmins have to deal with common tasks. The old ways may no longer suffice. Many books will be written about RHEL 6.
If the book doesn't cover the new RHEL 6 concepts -- for example PolicyKit, which has been around since F9 and is an important new technology to learn for system administration -- then it might have limited value as a practical guide to people who are using newer distros. But if it isn't meant to be a practical guide anyhow, then that's cool, too.
Just remember that a lot of the new technologies growing up in current Linux distros are evolutionary spinoffs of UNIX design. Some of them fit perfectly into UNIX philosophy, while others suggest a new direction -- sometimes Windows-like, and sometimes a unique new design altogether.