My absolute favorite book of his is 1983's collection Fire from the Wine Dark Sea which, in addition to one of the Roman stories, contains poetry, two biographical interviews, and one piece of sheet music -- in addition to a host of wildly creative short stories that span the topics of roller coasters, creepy twins, dancing on the surface of a star, and an alien invasion that results in something similar to the movie Groundhog Day (but with two alternate endings).
Somtow also followed in Tolkien's (and Barker's!) footsteps and created some beautiful new languages for his fictions. I couldn't find good online examples of the language (and the flowing, Tengwar-like alphabet) from his Inquest novels. Even more creative, though, was his graphical attempt to render the telepathic communications of whales. From his novel Starship and Haiku comes the following passage, which I've attempted to reconstruct as exactly as possible with Powerpoint...
"And he shouted out another word in the true speech of the whales, words that for humans can only be suggested as clumsy pictographs, a word of melding, trying to break them free of their cages---
Even though I know virtually nothing of his novels from roughly 1990 to present (which include vampires, other historical periods, and more contemporary/horror themes) and even less about his more recent domination of the Thai classical music scene (he serves as the artistic director of the Bangkok Opera and the Siam Philharmonic Orchestra), I can say without reservation that I just love this guy's creative mind and heart.
2015 update: More on Somtow's flowing Inquest alphabet here!