Well, the issue though with the current z80 ASM tutorials is that none were written with visual people in mind, neither people who have some reading comprehension problems like me (example: at school I had 75-90% scoring in everything except french reading comprehension and gym classes. Even in french essays I had higher score). You'll be given examples of code that absolutely have no concrete use at all, and be told to understand these, then have to read a "TL;DR" text about it. Everyone is different. Some people only learn from concrete examples. This is why it took me 1.5 month to get the grasp of most essential TI-BASIC functions: the TI manual barely had any concrete example of useful code and that's all I had access too (no internet and computer at home). The only reason why I managed to learn is is because it's closer to english language than assembly is and the TI manual BASIC doc was written in small sentences, no massive text to read. I also have trouble understanding other people code even in BASIC, so even reading source code doesn't help me much.

Very interesting point... I hadn't thought of that.

Btw, ztrumpet: day 15 ("advanced" math) is the hardest lesson of the tutorial after 23 (interrupts). I skipped both of them, at first, and didn't get back to them until way later when I already had a fair grasp of the language. (cheating works  ) You pretty much don't need the routines described in day 15 to understand the rest of the course. They come in handy when you start writing code independently, though.
(Personally, I never made any serious use of interrupts...)