Like with any computer system, it's important to have the proper documentation so that one can understand the architecture as well as create the appropriate hardware or software for it. PC-9800 series computers are generally well documented, it's just that information is scattered around the Web, and a lot of it is exclusively in Japanese.

The site at https://printf.neocities.org/programming provides a comprehensive collage of technical documentation, tools, libraries, and source code snippets.
Another useful page is at http://radioc.web.fc2.com/column/pc98bas/index_en.htm, detailing a lot of things regarding system setup, operating system quirks, and expansion boards, but the most important section is "Technical Information for developers". It provides information on disk interfacing, port mapping, memory mapping, and disk boot records.

In regards to more formal literature, there's:

Volume 2 is easily reachable and is the most comprehensive source of information for working with I/O ports and the memory map, but Volume 1 is rather difficult to find, with the only source I'm aware of being the previously liked archive.org page. I should probably upload all the text files here and fix up the encoding.

MASTER.LIB is a great library for getting started with Real Mode development, as it provides functions for various fundamental operations that deal with keyboard and mouse inputs, graphics operations, beeper control, etc. It's manual is not only a good form of documentation for the library, but also the 9800 platform itself.