z/Architecture Principles of Operation
Often referred to as the POP, from the title, this might sound like a pretty frightening book but if you want to write assembler, you need one of these by your side, or at least available to you.
From a programming point of view, chapter 7 contains information on all the general instructions. Chapter 8 covers the decimal instructions and chapter 9 covers floating point instructions if you need that level of precision. To be honest, in thirty years or so, I think I’ve only used floating point instructions a couple of times so basically chapters 7 and 8 cover pretty much all the instructions you need to know for general programming and you’ll be surprised how few you actually need to use when coding.
The early chapters also cover the general Z hardware architecture but my advice is, don’t try to take all this in in one go, just use it as a reference for instructions when and as you you need it and don’t worry about anything you don’t understand, it will become clearer as you write more assembler code.
If you Google z/Architecture Principles of Operation, you’ll find recent copies, usually on the IBM web site.