The Structure and Interpretation of Computer Programs (SICP).
About halfway through the book, they build an explicit dispatch table, and I instantly understood 1) why my C code was so crappy, and B) why C++ and OO languages miss the boat (hint: they only implement the dispatch table in one orientation)
Worth struggling through, even if you never write a line of Lisp code. It has a lot in it about nice designs, abstraction barriers, and how languages actually work.