Functional Programming

A delightful introduction to functional programming, particularly from the Lisp/Scheme perspective, is the Lambda Papers.

Combinatory Logic

The lambda-calculus is a well-researched and -understood way of describing computational systems. However, it only provides one perspective on functional programming; combinatory logic, another.

• The Wikipedia entry is a good place to start on what combinatory logic is and how it relates to the λ-calculus.
• The book To Mock a Mockingbird by Smullyan gave combinators bird pseudonyms, and the bird names have become part of the combinator venacular. Chris Rathman's Combinator Birds site lists the common combinators with their various representations. I have copied his table into an Excel spreadsheet as an alternate view.
• David Keenan provides an animated graphical representation of combinators(modelling the 'bird' and the 'bird song', as it were) at his site To Dissect a Mockingbird. I find his appendix on logic and arithmetic especially helpful.