I adore classic arcade machines. They're interested to look at, fun to play and designed for a different era of hardware and software design. While I've certainly spent some time poking around some of the FPGA code for classic arcade cores, I'm still pretty novice when it comes to the low level technical specifications for these machines. The brand that I have the most fond memories of is the old Capcom machines.
Fabian Sanglard has written just this amazing book on the internals of those early Capcom classics like Street Fighter 2. Going into extreme detail on the legendary CPS-1 board, you get more information than I've ever seen before on how it worked, how arcades at the time overall worked and every gritty detail in-between. It's a joy to read with amazing art and diagrams.
The book is available for a pay what you want at his website. His system for generating the book is also pretty clever with the source code available here.
There's also a great post about the PS2. Both were a lot of fun to read. It's always a blast to learn more about these machines that I spent hundreds of hours with as a kid. Link to Playstation 2 Architecture