Z80 ASM is both quite limited (no multiply or division) and very straightforward. That's how I remember it, at least :) For fond memories, dig a copy of Rodney Zak's "Programming the Z80".
The first and still the best programming book I ever read. It taught me architecture, instruction fetching and decoding, how the clock comes into it, as well as the usual assembly programming. I loved that book. I still regret throwing it out, 20 years later.
I don't know if it's because I'm a stupid old fart, but it seems to me that the explanation this book gives of microprocessor architecture, busses, etc are still unbeatable because Z80 is so simple. It's probably still a valuable introduction to computing, if only for the first few chapters on binary, computation, etc.
