I had a lot of the Usborne books. I knew of the existence of the machine code one from the backs of the ones I had, but no bookshop ever had a copy. I finally bought a copy in a second hand shop a few years ago (I haven't actually read it though!)

I had a Spectrum in the late 80s. I learned the basics of Z80 programming from An Introduction to Z80 Machine Code by R.A. and J.W. Penfold - published by Bernard Babani, well known for their electronics books. A family friend (more into electronics than programming) who had given me the Spectrum, also gave me some Babani BASIC books. I guess that's how I knew about the existence of the Z80 book, and got a copy of that.

I wrote a simple text editor for the Spectrum in BASIC, but all the checks as to whether you had pressed a key (combination) with a special meaning (arrow keys etc.) meant that it couldn't keep up with even fairly slow typing. (The code in question was a big sequence of lines of the form IF <this key was pressed> THEN <GOTO or GOSUB (can't remember which) to somewhere>. I'm wondering now whether you could make a faster dispatch mechanism in Spectrum BASIC instead of this naive solution.)

So I wrote an assembly routine just to deal with this bit. I didn't have an assembler. But, the wonderful Spectrum manual contained this table:

https://worldofspectrum.org/ZXBasicManual/zxmanappa.html

which I used to hand assemble my program. (I don't think I'll ever forget that ret is 201). When I gave away all my Spectrum stuff a few years ago, I kept the manuals.