The books are sold in EPUB, Mobipocket, LRF, PDF, and PalmDoc formats, and the purchase price includes a copy in one of these formats, as well as a free bonus of a PDF map from the original print editions. (Given that it is sold without DRM, of course, one format is really all you need.) Duane only accepts payment via PayPal for now.
The books are also available at Smashwords, though Duane will take in more money faster from copies purchased directly from her.
I already have these books in print, but I might just have to get the e-book editions—in part because I seem to recall hearing that Duane went back to the series and did some further revisions lately, though I cannot find any links citing just how extensive these changes were—even Wikipedia doesn’t mention them.
The Middle Kingdoms books are some of my favorites of Duane’s. They borrow similar themes and essentially the same deities from her Young Wizards series, though they are clearly set in a different universe (so just call it the same universal structure). They do deal with some fairly adult themes, including matters of alternative (at least, in this universe) sexuality, which may be off-putting to some.
The only problem with the series, from my perspective, is that it has not yet been completed. A fourth book, The Door into Starlight, has been on Duane’s agenda for at least a couple of decades, but she has yet to write it. As she said in a recent Twitter posting, “Starlight is on the agenda, believe me. I know how it begins. I know how it ends. BUT OY THE MIDDLE. (headclutch)” Hopefully she’ll figure it out sooner or later.
In addition to the Middle Kingdom books, Duane also sells her Raetian Tales book A Wind from the South from her e-books page—similarly inexpensive and DRM-free. (She also links to six of her Star Trek e-books at Simon & Schuster, though they are more costly and presumably DRM-encumbered.)
In December 2006, I interviewed Duane on my talk show podcast The Biblio File about her plans for the Storyteller’s Bowl book The Big Meow, and she also called in during my January 2007 interview of Peter S. Beagle. It’s great to see her selling some of her own backlist books in DRM-free formats, and I hope she has a lot of success with that format.