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Learning Information Architecture
Don't Make Me Think!: A Common Sense Approach to Web UsabilityDon't Make Me Think!: A Common Sense Approach to Web Usability by Steve Krug
Buy new: £14.94 / Used from: £11.52
This isn't strictly about IA but if you are completely new to the field it is best to get a bit of context on usability. And this is a simple easy read.
The Elements of User Experience (Voices (New Riders))The Elements of User Experience (Voices (New Riders)) by Jesse James Garrett
Buy new: £10.83 / Used from: £13.42
Explains one perspective on where information architecture fits into the user experience field
Information Architecture for the World Wide Web: Designing Large-Scale Web SitesInformation Architecture for the World Wide Web: Designing Large-Scale Web Sites by Louis Rosenfeld
Buy new: £13.84 / Used from: £12.15
You have to read the polar bear but I wouldn't necessarily start here particularly as it is 504 pages
Communicating Design: Developing Web Site Documentation for Design and PlanningCommunicating Design: Developing Web Site Documentation for Design and Planning by Dan Brown
Buy new: £17.39 / Used from: £21.43
Rightly or wrongly, IAs are often measured on the quality of their documentation. This is a good intro to all things documentation