The latest additions to the PC Emulation Book:
A reference for the BIOS Data Area
https://book.martypc.net/appendices/bios-data-area
IO port reference for common PC/XT/AT hardware
https://book.martypc.net/appendices/io-ports
A handy reference for CGA/EGA/VGA palettes
https://book.martypc.net/appendices/video/palettes
I've also started documenting the EGA register file, since I've been all up in the EGA recently.
https://book.martypc.net/appendices/video/ega-registers
An EGA-specific reference will be handy as if you've ever wanted to emulate an EGA you often find yourself sorting through VGA references and removing all the VGA specific bits.