Programming Game AI By Example by Mat Buckland is an excellent resource to get started. He starts right from the basics of state machines and moves on to steering behaviours and graph theory for pathfinding. Probably one of the best resources I've found, especially when starting out.
The AI Game Wisdom books are also very good, but a lot of the articles are pretty advanced.
A good link for steering behaviours is this site: http://www.red3d.com/cwr/steer/
And as has been mentioned, http://aigamedev.com has amazing content, right from the frontlines.
Amit's Game Programming site is pretty decent at getting an overview (although admittedly, I haven't used it much): http://www-cs-students.stanford.edu/~amitp/gameprog.html#ai
I'd thoroughly recommended Buckland's book though. Very easy to follow and the source code actually works :)