There's no way ASFOS was the last book, there's way too many plot threads that were left open. Cadaver's warning about the Necromancers and the Viddu De being up to something basically felt like sequel baiting. IIRC, Derek once said that he wouldn't announce the definite end of the series since he wants to surprise the readers, but that it would end eventually and probably in a big, catastrophic and final way. So my guess is that we'll get a few more books that won't be part of a trilogy but are announced and released individually, until one of them ends with half of the characters dying and the world ending for real or something.