I just wanted to recammend this book. It's basicaly a satire of Mulan (girl disguising as boy to join an army) with an interesting perspective. While it isn't technically about transgender characters, it offers an interesting insight to how men an women are viewed by the opposite gender. It also has plenty of amusing twists and turns, especially towards the end. I loved this book. Of coarse, I love almost all his books :). You can find it in the sci-fi/fantasy section of any bookstore.
I love Terry Pratchett. Haven't read that one, but I'll keep an eye out for it.
Dennis