What OS to use to learn Unix?

Yikes! Is that book still in print?

It shouldn't take too much to put one of the BSD Unix flavors (FreeBSD, OpenBSD, NetBSD come to mind right away) on there, to get a Unix OS.

One of the popular Linux distributions will probably work as well. I like Scientific Linux, a Red Hat Enterprise Linux variant put together at CERN to provide scientists and staff there with a common basic software base. Mint, Ubuntu, OpenSUSE, are also modern and maintained. The only issue

I foresee is that the bash shell that's commonly used by just about everybody is based on the Unix "Bourne" shell, and your book might use one of the others. Free versions of the C and Korn shells are available and usually present, so that would just be an extra step to either launch the proper shell for exercises, or change your login profile to default to the other shell.

Best Unix Os

Ubuntu is a good start like other answers said, but if you want to get deeper, you will eventually want to migrate to Arch or Slackware. Even Gentoo (total source based, but has an installer) would be a task if your not used to Linux.

Debian/RedHat (or their add ons) lets you use Linux, Arch/Slackware will teach you Linux.

All in all though, if you get comfortable with the command line using a couple different shells (BASH,TCSH,KSH) you will be doing pretty well.

True Unix isn't used that much anymore, everything has nearly moved to the BSD/Linux roads. BSD is closer to Unix, but I prefer the Linux road, just a preference though. Anyways...Have fun with it.

Your best bet would be one of the Free BSD variants http://distrowatch.com/search.php?ostype=All&categ... or even OpenIndiana http://distrowatch.com/table.php?distribution=open... which are UNIX-Clones.

In terms of Linux, most will do, though I believe that Red Hat (CentOS, or SL), Slackware, and Gentoo would be closest in being compliant to UNIX.

Almost any flavour of Linux. Pick a popular one. Ubuntu (as above) is a good choice.

Ubuntu