This is a great book to introduce business people to information architecture, for architects to reinforce their skills, and for web designers to principles to apply to site design. The second edition has more information and is more in depth than the first, and is well worth purchasing.
The first three chapters of the book explore what information architecture is and what it is needed. Chapters 4 - 9, the "Basic Principles of Information Architecture" have the most substance. Several chapters bear reading several times, including:
Chapter 5: Organization Systems, Chapter 7: Navigation Systems, Chapter 8: Search Systems and Chapter 9: Thesauri, Controlled Vocabularies, and Metadata
The sections on Process and Methodologyactice, and Organizational fit are all good for people learning about IA, but may be too basic for anyone that does a lot of work or reading in the field. The Education Chapter is already out of date, which is to be expected.
IA for the World Wide Web is a great book, worth reading and worth hanging onto for reference or to use to explain the IA to others.