What do you think would make a better target? C maps pretty closely to assembly, so it seems like it would be the simplest. Maybe Pascal or BASIC, but most people these days don’t have experience with Pascal, and BASIC would probably be too simple for a full-length book.

For writing an interpreter or transpiler, there are probably better options, but for a true compiler I can’t think of a better choice than C (or at least a subset of C).