McQuillen and Prince's "MVS Assembler Language" has a clear sense of direction and is well written. It provides a good introduction to S/370 assembler language for the uninitiated. However, advanced students may find that it falls short from a technical point-of-view; it fails to mention the system 370 PSW, privileged opcodes, interrupt mechanism or the channel subsystem. I wish they had covered reentrant programming techniques. Coding for reentrancy in assembler language is a pervasive discipline, and really belongs at an introductory level. Consider this book if you have been charged with responsibility for maintaining someone else's assembler language code and you haven't a clue where to begin. But if you plan to do development (and especially systems programming), then this book will only take you so far.
The best reference manual is one that's filled with examples, not only dry text. This one does that. It's helped me many times in my job as independent contract programmer. I bought this book for myself and use it on every assignment.
This book is a very good reference guide to IBM Mainframe Assembler for those with considerable experience in Assembler. Other experienced programmers without Assembler would need to get more pre exposure to the basics for this book not to get too deep, too fast.