Some people are so used to bad Oracle books they honestly can't imagine what a good one would be like. This book is well written and clear. The author has a complete grasp of the subject.
Before purchasing this book, however, you should know what it covers -- the internal architecture of Oracle. This is a book for Oracle DBA's. It is not a quick overview. The author provides a fairly complete picture. Because it is well written most people will understand the material right away.
You should also know what this book is not. It does not get into the nuts and bolts commands a DBA will use on a day to day basis.
Although it is tempting to say this book deals with theory, it is actually practical. Every Oracle DBA needs to know what is in this book. Too many Oracle DBA books jump into detail without providing a grounding in how Oracle works. This is the book to have if you want to see what's going on inside Oracle.