Earth, the Musical and Googol, the Musical are companion novels, but each stands on its own. You can read one without reading the other (but why would you?). ETM came first, by several years — not for lack of ideas, but simply because Jan St. Louis has a full-time job as a magazine editor and writer, as well as a house requiring regular maintenance.
Googol, the Musical is new on the market, so reviews are pending. But here's what some readers had to say about the earlier book: