I am looking for a book on the symbolic/mythological meanings of the planets - any suggestions?

I doubt you wont find this info rolled into one book. There's European, Celtic, Tibetan, and Greek, among others... even Native American Archaeoastronomy... I went to a seminar taught by Dr. Robert Fleck, who was in the process of writing a book, but I don't think it's been published yet. He is currently a professor at Embry Riddle Aeronautical University in Florida.