These are not the same sort of book. Witnesses to a World Crisis is an excellent volume, but most of its bulk is made up of a study of the sources which describe the conflicts in the seventh century. It is an academic book, and while Howard-Johnston provides a narrative account at the end, this is probably not what you're looking for. That said, I can't really recommend The War of Three Gods since it's just another middling retelling of the story, although not as bad as Reagan's First Crusader: Byzantium's Holy Wars, which largely deals with the same topic. Still, I think you're better off reading Witnesses to a World Crisis: Howard-Johnston has a more nuanced approach to the wars and is less focused on battle (which I'm not convinced was terribly important during these campaigns) than Crawford.