First, http://www.skotos.net/articles/ Skotos is an incredible and excellent resource. It's also a little bit dated, but I was reading it rabidly in 2001-3, and it gives you some very powerful and useful ideas. It leans to the side of the technical, but it's also the place that taught me that my interest in virtual worlds was more importantly an interest in communities.
Second, the MUD-Dev mailing list archives. If you can get that to work, let the world know. Particularly me. I learned a lot by reading the posts off the mailing lists, and a few times, I even posted back. Good times. The Terra Nova blog was started primarily by an announcement on the MUD-Dev mailing lists, and while I dismissed it at first, the entire population seemed to migrate over there, effectively extinguishing the usefulness of MUD-Dev.
Third, personal sites, like Raph Koster's essays, Richard Bartle's Website, which includes the famous HCDS, Mike Rozak's Deeply Random Thoughts, Christopher Allen's Life with Alacrity.
Fourth, play the games. Read the websites. Look at screenshots. Read user experience. Timothy Burke became famous precisely because he wrote that grand essay on how much Star Wars: Galaxy sucked. Of course, he then proceeded to write an actual academic piece, which is probably why they justified inviting him as an author to Terra Nova. =)
Fifth, the hardcopy books. The most key one is Richard Bartle's Designing Virtual Worlds, oft cited as DVW. Jessica Mulligan published one that I couldn't stomach, Developing Online Games: An Insider's Guide, and Edward Castronova has a book called Synthetic Worlds, which I will be receiving in the mail later this week. Further, there is Raph Koster's Theory of Fun for Game Designers, though I think it's theoretically applicable to designing anything to be 'fun'. Because virtual worlds is such a new field, there isn't really a strong set of literature backing it (as many academics have lamented). Instead, you can pick up a lot if you look for sociological and ethnographic studies of online communities, communities as a whole, psychological notions. Bartle's DVW was the reason I read Joseph Campbell's Hero With A Thousand Faces; Campbell's overview of myth and his Freudian insight into the human individual-communal character was a tremendous boon in my own philosophy. I've never read Dibbell's My Tiny Life, but I suspect I should.
Names you should always notice when they crop up: Allen, Bartle, Castronova, Dibbell (that was just too easy..), Koster, Sellers, and Nick Yee. That's all I can remember presently, but there are probably a few more strong players in the industry/academia. And yes, I did keep the lawyers, Lastowka (his name disappeared from TN.. I wonder why) and Hunter, off that list intentionally. Granted, Hunter has been putting in more visibility by commenting in papers and such and then posting them to TN, but *shrug*.