For those new to Information Architecture and the role of the IA, UX Booth has written an introduction to the field and role, by taking a look at how some noteworthy IAs have defined the discipline, and looking at some of the work that is typical of our craft.
It can be confusing to understand the role of the IA, because in many cases, IA encompasses more than what is defined in this primer. Depending on where you work, especially if you work in smaller teams, the IA role has a tendency to overlap other disciplines and do research, user testing, and interaction design. But UX Booth has provided a somewhat purist definition of the theoretical role which is very helpful. JJ Garret's Elements of the UX provides a good description of where IA fits into the bigger picture and Rosenfeld/Morville's Information Architecture for the WWW or Christina Wodtke's Information Architecture are excellent places to get to know the nitty gritty of IA work.
http://www.uxbooth.com/blog/complete-beginners-guide-to-information-architecture/