There are many classics of functional programming that can help you take your thinking to the next level. My recommendations are for making you think in a new way.
- [Structure and Interpretation of Computer Programs][SICP] (SICP)
- [Lisp in Small Pieces][LISP]
- [Paradigms of Artificial Intelligence Programming][PAIP]
- On Lisp
- [To Mock a Mockingbird][mock]
What are your recommended classics?