Functional programming

Quote from Wikipedia:

In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions, in contrast to the imperative programming style, which emphasizes changes in state.[1]

In practice, the difference between a mathematical function and the notion of a "function" used in imperative programming is that imperative functions can have side effects, changing the value of already calculated computations. (...) Conversely, in functional code, the output value of a function depends only on the arguments that are input to the function, so calling a function f twice with the same value for an argument x will produce the same result f(x) both times.

Lisp e-books

• Practical Common Lisp by Peter Seibel.
• Common Lisp: A Gentle Introduction to Symbolic Computation by David S. Touretzky.
• Succesful Lisp by David B. Lamkins..

I've started with gentle --actually a really old book--and have planned to read pcl second. [Edit 23-11-2010, now actually bought a used copy as well!]