https://www.amazon.com/Dissecting-DOS-Code-Level-Operating-S...
The original companion floppy for that is available on archive.org:
https://archive.org/details/dissecting-dos-companion-floppy-...
Understanding Microprocessors is a small book, and currently unavailable anywhere I've seen except for eBay, but it's also normally quite inexpensive.
Actually there is quite a bit already for a single person to digest, if you want to understand the real details of how everything works. I learned about the DOS family by reading this book:
http://www.amazon.com/Dissecting-DOS-Code-Level-Operating-Sy...
UnixV6 and the Lions' book come close, but Unix is still far more complex than DOS because of its multitasking, multiuser heritage. Early Unices may be within the realm of an average student to grasp, but things like the BSD and Linux kernels are orders of magnitude bigger and more complex, which means that it's nearly impossible to get down to the details when studying them and the best thing one can hope for is a high-level overview of the various subsystems and how they interact. DOS is simple enough that a single person can study the whole OS at the level of individual instructions (and write an OS similar to it).
https://www.amazon.com/Dissecting-DOS-Code-Level-Operating-S...