I'll take the easy question first: Ghostwalk was a Mark Ellis book.
As for the other question: It is not accurate to say that he is quitting. He has at least one more book currently contracted, and there is no telling what contractes may or may not be offered and/or accepted in the future. Mark has several other projects happening, some publicly announced and some not yet at that stage. He has his own website at MarkEllisInk.com where you can get direct answers from him.