This was one of the first books I got for the Atari machine. Unlike an Apple, which came very well documented and was a simpler hardware design, the Atari had custom chips with some better capabilities, but very little in the way of information.
Spent a lot of fun days reading that book, then writing little programs to learn to exploit the hardware. Happy days.
A nice reminder of the days when hardware was far simpler and in some ways more "idiot-proof"... accessing undocumented device registers on a modern system could in the worst case cause irreversible damage, e.g. by overvolting various components including the CPU.
Spent a lot of fun days reading that book, then writing little programs to learn to exploit the hardware. Happy days.