That's a long time ago. ;-) As I recall, there was a set of 2 or more books, published in the early 90s. And I never had my copy, but several of the software devs had them on their shelf, and I'd borrow them as needed. Typically, my use was to write a binding for Visual Basic. For instance I hacked into the audio hardware, and wrote a bespoke interface to the serial ports.

It was strictly API stuff, not how the actual OS works inside.