I still have my original copy of Zaks' book in my collection. I obtained it in 1981 from a friend. I did so much z80 assembly on the TRS-80 that I could "see" instructions in a hex dump before asking zbug to disassemble them.

The z80 was quite the chip for its time and I enjoy reading these articles has they look back into the deeper inner workings of the chip.

One could master the z80, I'm not sure many modern processors could be mastered at that level by a single programmer. They seem to have gotten so complicated that they're beyond comprehension in a single person's mind. I could be wrong but that's the general feeling I get. Sure you can be really good at them and maybe an expert in many applications of a modern complex processor but I doubt many people have a singular understanding of everything each chip has to offer.

Or maybe I'm just getting old.. :-)