For beginners, I'd really recommend Mat Buckland's Programming Game AI by Example
It is very easy to follow and I've found that it provides an excellent starting point for Games AI.
However, as the posters above have previously said, the field of game AI is incredibly broad and more and more academic AI techniques are being introduced as the hardware advances. The AI Game Programming Wisdom books are very, very good, detailing techniques from various real world examples.
Ai is usually very game specific and very hard to wrap in a reusable "Plug and play" sort of system. The AI libraries that do exist contain common things such as some mathematics, prediction, flocking, and path finding. I don't know of any libraries in Objective-C that cover this, but your game sounds like it would not really benefit from these because your AI requirements are fairly simple.
The best way may be to write your own AI from scratch. Start by breaking down the thought process for your AI into some kind of state machine (Make trees on paper, list steps, etc) and write it step by step.
Ai is not a fast moving field like graphics. Something from 2004 in terms of AI should still be very relevant. I would take a look at that library you talked about as well.
Here is an example in Objective-C on A* pathfinding:
Make a post on the Gamedev AI:
I strongly recommend reading this AI book:
This book will teach you how to think through the AI process and will teach you how to write several systems that could become a part of an Ai framework.