"Concepts, Techniques, and Models of Computer Programming" by Peter Van Roy and Seif Haridi. Also "The Reasoned Schemer", which has the same pedagogical style

"Concepts, Techniques, and Models of Computer Programming" by Peter Van Roy and Seif Haridi. Also "The Reasoned Schemer", which has the same pedagogical style as the other Schemer books, but works around logic programming. Both of those books are mind benders and I've gotten a lot out of them recently.