When I started learning about BI (Business Intelligence), a few members of the Pentaho community advised me to read this book. I'm glad I did. Kimball is one of the "fathers" of data warehousing, and his book had a lot of great insights for dimensional modeling. It helped me avoid many design mistakes while building my DWH, and gave me insight I might have taken years to discover.
It's a "theoretical" book, in the sense that it does not focus on any specific technology; it's also a "practical book", because he uses real-world scenarios (inventory management, e-commerce, CRM...) to demonstrate the various dimensional modeling techniques. I also liked the part about BI project management and encouraging BI in a company (= how to engage users and how to "sell" a BI project to management).
He also has a newsletter with many DWH design tips (archives here: http://www.kimballgroup.com/html/07dt.html ).