
This text provides a comprehensive treatment of virtual world design from one of its pioneers. It covers everything from MUDs to MOOs to MMORPGs, from text-based to graphical VWs.
And another recommendation if you're interested in worldbuilding is the Encyclopedia Eorzea books from the Final Fantasy XIV game, three of them; they contain the full backstories, timelines, summarised in-game stories, locations, characters, races, etc etc etc. However, it's written "in-universe", not so much from a developer's point of view.
That said, you didn't specify which part of said games you would like to read about, I can imagine more information about the design and build would be interesting too, a look behind the scenes. The video game resources I mentioned get you there partway though.
The Art of Game Design: A Book of Lenses, by Jesse Schell
Level Up! The Guide to Great Video Game Design, by Scott Rogers
A Theory of Fun for Game Design, by Raph Koster
Designing Games: A Guide to Engineering Experiences, by Tynan Sylvester
Game Feel: A Game Designers Guide yo Virtual Sensation, by Steve Swink
Designing Virtual Worlds, by Richard Bartle
Here are a few more that I haven’t read and don’t own, so don’t know anything about them:
Elements of Game Design, by Robert Zubek
Advanced Game Design: A Systems Approach, by Michael Sellers
Game Mechanics: Advanced Game Design, by Ernest Adams & Joris Dormans
The Ultimate Guide to Video Game Writing and Design, by Flint Dille & John Zuur Platten
I’m sure there are more. You have quite a few to choose from! If you plan on reading just one, I recommend the first one on my list.
Awesome site by the author (a legend) where you can download the PDF: https://mud.co.uk/dvw/.