I have mixed feelings about this book. It fills an important gap in the market and as someone who uses DB2 daily I feel I should support any books on the subject. However, this book is let a down in a number of ways. Most obviously, there are many errors in the text. Often examples contain syntax errors - a particular issue for me trying to use it to understand xquery for the first time. I asked the publishers if they had a list of known errors in the text but they don't.
One of the big selling points of the book is that it contains sample questions at the end of each chapter and at the end of the book. Well, some of the sample questions are poorly phrased, and one simply gives the wrong answer - including an explanation justifying the incorrect answer. Also, the questions at the end of the book are bulked out by the questions already posed at the end of each chapter, so there are significantly fewer questions than you would imagine.
Much of the text is lifted directly from the IBM web site, so if you go looking for an alternative explanation of a subject that you find hard to follow, you just get the same words from IBM. To me, a significant reason for paying for a technical book is to get an alternate explanation of a subject.
You can get 7 pdf documents from IBM for free that cover the same material. If you're on budget you might be better off using these.