What would you recommend me to read if I want to dive deep into the functional programming paradigm? There's plenty of this kind of material for Object Oriented programming (of course, given the hype.), but i dunno about FP.
The obvious recommendation for fp (functional programming) on HN is SICP (Structure and Interpretation of Computer Programs). The full text is free online http://mitpress.mit.edu/sicp/
Everyone raves about SICP, from top to bottom of the fp world.
However, some objections: 1. it purports to be an introduction to programming, but it assumes a lot of background knowledge (so that's fine if you have the background, but the inconsistency irritates my inner pedant). 2. the authors don't use it themselves anymore in their programming course (they switched to python, for variously debated reasons).
What kind of background knowledge is required?
There's also a bit of imagery that only makes sense if you already know what it means. As someone who's taught at Uni, I think that's pretty poor in an "introductory textbook". However, the simple truth is that it isn't an introductory textbook at all, but something more advanced and arcane.
One approach I took was to go to the (University) library, and find the section on the programming topic of interest. Then grab a few books, and read a paragraph at random. Then borrow (or read there) the one that appeals to you - the writing style that makes sense to you, that addresses the issues that come to you as you read, that communicates to you. You can do the same at Borders, but the books are more vocational and less textbooky.
If some particular part is unclear, you can compare it with other books - you now have a resource.
Every book has something to offer. The biggest factor is what you put into it. Therefore, it's more important to pick a book that you like rather than the "best" one. Otherwise, you can get into a kind of McCarthyism where there are good books and bad books, instead of each book having strengths and weaknesses.
To put this in perspective, I personally really liked "Java in a Nutshell", because Flanagan's writing is brilliant. It's very succinct, yet covers all the cases, has a joie de vivre about it, and yet is so transparent it's easy to not notice it (Douglas Adam's prose also has this quality). Revealingly, Flanagan loves writing; and is very well read himself. Most writers of programming books are not. Now, to me, being well-written is very pleasing; but I can imagine it not mattering at all to the next programmer.
It covers Standard ML, but you will find that the ideas of FP are easy to move around from language to language.