When you decide how a designed system is structured, and how people find their way around, that's information architecture. Having said that there are probably as many definitions of information architecture as there are information architects.
How to Make Sense of Any Mess
A short book that does a good job of teaching you the principles behind information architecture, with less focus on the practical methods you might use to do it. It's available online for free, so you've got nothing to lose.
Information Architecture: For the Web and Beyond
Often called "the polar bear book" because of the animal on its front cover, this is the information architecture bible. Goes into detailed methods, tools, and approaches to information architecture.