This book is one of the classics on business rules from one of the most long-standing authors in the area, Ron Ross. The book is a little more than three years old but, as it is not really focused on technology for managing business rules so much as the general approach, it has aged well.
Ron does a good job explaining what business rules and how to capture them and gives a solid overview of things like fact models, processes and how they relate to rules, and dos and don'ts of rule writing. There is a lot in the book about his particular approach to writing rules which, whether you follow it or not, has some good advice about usage and style for writing declarative business rules.
The book does not go into details on the technology of implementing business rules using a business rules management system but instead focuses on the value of an approach that separates business rules from other kinds of requirements and manages those rules as an asset. A good book to introduce the subject to someone without a technology bent.