HP-UX
HP-UX is HP's version of Unix for their servers. I've been developing software for HP-UX for the last ten years. If you want to develop on Unix and HP-UX there is some required reading. The first is Advanced Programming in the UNIX Environment which is written by the late W. Richard Stevens. Of course if you are going to do any then you need to also read UNIX Network Programming.
On the HP-UX System management side there is HP-UX CSE: Official Study guide and Desk Reference.