Update, August 20: Click here to download Designing Virtual Worlds as a .PDF file.
Designing Virtual Worlds is a milestone book in the history of the genre, relevant to basically anyone working in or thinking about online games/metaverses/MMOs/etc.; written by UK professor Richard Bartle in 2003, much or most of it is still relevant to today's technology. (Perhaps even more relevant now, as each wave of designers and developers insists on repeating mistakes made 10-15 years ago -- just on a much, much larger scale.) And Richard just announced he's offering it as a free download on his site:
It's held up reasonably well in some places but not so well in others. 18 years is a long time in virtual world development (the book predates World of Warcraft), but there may be something there of interest to people who haven't read the physical copy.
Click here to open Designing Virtual Worlds as a .pdf.
Professor Bartle, with characteristically wry self-effacement, just sent me his personal summary of the book chapter by chapter, noting which sections are still most relevant and which, not so much -- a handy cheat sheet for new/returning readers: