The Number of the Beast by Robert A. Heinlein
This novel is about four geniuses travelling through space and mutiple-dimensions in a flying car with its own AI. The travellers wander through multiple science fiction universes, including several of Heinlein’s own.
A fun ride for Heinlein fan, but I would not recommend this for someone reading their first Heinlein novel. If you haven’t read any thing by the Grandmaster of American Science Fiction, you are missing not just good adventure stories. As author Spider Robinson so aptly put it, “And I repeat: if there is anything that can divert the land of my birth from its current stampede into the Stone Age, it is the widespread dissemination of the thoughts and perceptions that Robert Heinlein has been selling as entertainment since 1939.”