You can read all the books you want, but until you setup a machine and break it a few times, you don't really get a feel for it.
There are plenty of different unix boxes and yes some are hardware dependent e.g. SPARC, but Free/Net/OpenBSD are all unices and run on x86-class [as well as many other] processors. Linux is unix-like, but not really unix, you wont learn anything from Ubuntu. Personally I love FreeBSD as far as unix goes.