He has finished writing the third volume, but (SHOCK) they have had to split the final volume into two parts, since it's too long to publish in one. The first half just came out in hardback about two months ago. The second half will be out later this year. This is all in English, US and UK versions. (I don't know if you read these books in English or wait for translations.)

It's a sidhe/fey story, set in an alternative Europe. Tad Williams seems to be more fascinated with the fey than any other theme in his tales. And there are more lovely catacombs, but that is about where the similarities to MST end. IMHO it's a better series so far that MST -- he has matured a lot as a writer. But that's just me.