Roughly speaking, there are two kinds of computer professionals in the world: "client/server guys" and "mainframe guys." There are a lot of the latter who are trying to learn what they need to become the former, but there aren't very many client/server guys trying to learn about mainframes.
However, for those few (like me) who started out in client/server and have only the vaguest notion of what the mainframe environment is like, it can be very frustrating to find good books that can help you. Most of the mainframe references you find in bookstores are geared toward existing mainframe guys.
"Illustrated SNA" is the best introductory book to the mainframe environment that I've seen. Naturally it focuses on the networking aspects of mainframes over all other facets, but it necessarily touches on those other facets and in the process defines all those freakish acronyms you see in the want ads: CICS, MVS, VTAM, etc.
The book is disarmingly short, but the content-to-verbiage ratio is pretty high. It's very understandable and littered with powerpoint slides (also included on CD). The CD is useless, to be honest, but the rest of the book is worth the price just so you can crank your mainframe comfort level up a few notches.