“The Last Witchfinder” is a novel with two different authors, only one of whose names appears on the cover. He is James Morrow, a writer whose reputation has not caught up with his accomplishments. Mr. Morrow is a very odd bird, a novelist of intellect, bravado and wildly phantasmagorical imagination. In the eight works of fiction that precede this one and have sometimes fused satire, philosophy and science fiction, he has, among other things, envisioned a literal Godhead the size of Delaware.