Excellent book. I actually used it for over a year to solve many performance problems on our prodution databases. I have actually gone beyond the book's level to solve prodution issues. Let me explain: the book is excellent, it gives some simple examples of query so one can understand the underneath wiring. However in production performance tuning scenarios, I have most of times more complex situations that I have to think outside the box and utilize author's concept to solve real-world problems. It is an excellent reference book to prove to your boss what was wrong in the system.
This is a must read. BTW, another "must read" book is Tom Kyte "Expert Oracle ....". He wrote for Oracle, but the TP explanation (isolation, locking ....) is excellent (if anyone have questions or doubts how database processes records, you must read this book).