https://psx-spx.consoledev.net/ - This is a conversion/edition of Martin "nocash" Korth's Playstation specs document originally hosted at https://problemkaputt.de/psx-spx.htm
http://psx.schnappy.xyz/starting Everything to get you started in PSX dev on one page
https://schnappy.xyz/nolibgs/ - Wiki that documents data conversion and usage for PSX formats.
MIPS Instruction Set - The MIPS instruction set for the PlayStation 1 R3000A CPU.
Padua's Playstation 1 Resource - A great wealth of technical info on the Playstation 1; very Assembly language heavy.
"Introduction to PlayStation research, presented by Dr.Hell"