[blackcompe]

I found a free e-book of The Implementation of Functional Programming Languages by Simon Peyton Jones.

At first glance the book looks very accessible and should serve as a good starting point for anyone with a fundamental knowledge of functional programming. This is not however a book on learning functional programming. It's more geared toward learning the theoretical foundations of designing and implementing language features (e.g. Pattern Matching or List Comprehensions).

There wasn't an appropriate sticky to put this in, so I stuck it in the Scala resource thread for future reference. Enjoy!