One of the things I like about "retro" computing are the books of the era. There were a ton of books of varying quality, but the good ones really dove into the internals of what was going on with the machine. "Mapping the Atari" by Ian Chadwick was one such book where Ian spent a lot of time checking to see what various peeks and pokes did on the Atari machine (Atari didn't see fit to provide a lot of documentation about the machine unless you gave them some serious cash). Conversely the Commodore and Sinclair machines were well documented by their creators. This allowed for more in-depth books about the machine (and even books with the complete source code of the machine, such as Complete Timex TS1000/Sinclair ZX81 ROM Disassembly and The Complete Spectrum ROM Disassembly.
Understanding Your Spectrum is a fantastic book that I wish was available (much like the ZX Spectrum itself) in the USA when I was learning. It not only covers the basics of the machine (what hooks to what, what the board layout is) but then explores the operating system of the machine. I don't recall this level of depth in many books that I saw in the wild when I was growing up , and I'm not sure I would have known what to do with them (I didn't have nearly as much knowledge about computers as I do now) but I would have appreciated at least this level of detail. It really pulled back the mystery of what was going on in the machine.
It got me thinking that many of today's machines would benefit from this deep-level dive. I know there were books that gave a cursory overview of the internals of the machine but having an expert guide like Dr. Ian Logan who can explain what's going on with the machine would be a huge help. It's not that we need documentation; we need explainers to understand what we're looking at.
So here's to Dr. Ian Logan's "Understanding Your Spectrum", one of the great tour-guides that helped make the ZX Spectrum an exceptional machine.