alt Hacker NewsZ-80 was the processor for my introduction to programming in assembly language on a TRS-80 model I in early 1983. Bill Barden's assembly language books and Hardin Brothers' "The Next Step" column in 80 Micro magazine paved the way.
I wrote a quick post a while back about my Z-80 experiences here:
When I was a teenager ca. 1980, my local Radio Shack had a shelf of books, and I worked my way through them as my funds allowed. Since the TRS-80 used a Z80, they had Barden's book on the Z80, published by Howard Sams. I already understood programming a bit thanks to learning BASIC, and my curiosity about micro's was piqued by my mom's subscription to Byte Magazine.
Maybe it's because it was so clearly written that a kid could understand it, but it was a revelation, and I read it cover to cover. I never actually touched a real Z80, but it was a good enough introduction that I had no problem understanding other micro's including the 6502 and early microcontrollers such as 8051 and PIC.
Even today, I suspect that I "understand" modern micros by falling back on the Z80, and I recommend learning an 8-bit micro because they're simple enough for mere mortals to comprehend.