HP-UX is not free. The basics are the same as RedHat (ls, cat, cd, etc). However, the kernel, disk management, etc are considerably different. It also does not run on x86 architechture (depending on how you classify Itanium).
There are some good books specific to HPUX, unfortunately I can't remember the names off the top of my head.
I don't know if I can offer any good advice on how to learn HPUX easily. It was just a progression for me. I started on AIX and Linux in college. My first job sent me to Solaris admin courses, and my current position has Tru64, Linux, Solaris, AIX, and HP-UX - All of which I must know to some degree. Usually if you get good at one or two, the others aren't too different and you can figure it out.