Designing Virtual Worlds book is free for limited time [pdf] (mud.co.uk)

I downloaded this and intend to read it over the coming weeks - at 950-odd pages, it's lengthy.

Dr. Bartle is still one of the best thinkers on this subject. Also, his personal website is delightfully 90s-esque.

https://mud.co.uk/richard/

Free forever if if you use LibGen: https://libgen.rs/