Releasing a second (and probably more) tome seems to make sense if you take the time to notice the book description. It says "This is the eagerly-anticipated first book in a new series from Warhammer Forge". Maybe the dropped the original plan to make a mega book of 90+ monsters in favor of a number of books. So we should look forward to a Monstrous Arcana II and III (?), unless they change their plans in the meantime (tamurkhan was supposed to be the first of four books as I recall).