Books

Types and Programming Languages - Benjamin C. Pierce

Great book, helped me greatly in understanding the type systems, and in reading programming language specs in general.

Learn You a Haskell for Great Good! - Miran Lipovaca

The fun book to get introduced into basic haskell.

Real World Haskell - Bryan O’Sullivan, Don Stewart, and John Goerze

A lot of code examples, and ready-to-use code fragments. However, the version of haskell it describes is a bit outdated.

Conceptual Mathematics: A First Introduction to Categories - F. William Lawvere, Stephen H. Schanuel

A gentle introduction to very basic category theory with great exercises that go with the text.

Web tutorials

Typecassopedia

Introducing many of the typeclasses.

What I Wish I Knew When Learning Haskell

How I wish I had learned them!