After struggling for weeks in a local community college online class using Peter Abel's Programming Assembler Language book with the PC370 IBM370 emulator, I happened upon a link to Bill Quall's excellent book. It is extremely readable, has excellent, clearly documented & complete programs to illustrate concepts, and very educational end of chapter quizzes with answers in the back. Best of all, it is written specifically to work with Don Higgin's PC370 emulator (the differences are few, but important). I doubt I would have passed the class without this book (due to lack of applicable information in the resources I had prior to finding this book).
The only thing I could come up with (and it is only a caution, not a true negative) is that although the book is 563 pages, it does not go as far as I would like. I would love to have a volume 2 by the same author which describes use of multiple CSECT programs and MACRO writing (and whatever else I don't know I'm missing).
Another caution: I paid $25 or so for a used copy and now the only copy I see online is offered at $99 (used), so I might look around a bit or try to plow through the IBM Principles of Operations (free online) unless I absolutely needed to get up to speed quickly for class or work.
Overall, I cannot praise the writing style and educational value of this book highly enough, especially for a newbie to mainframe assembly programming (don't think it will be of much use to those wanting to learn Intel assembly!)