teaches the reader how it works and how to best approach a problem
For those who have the O'Reilly book "Programming with Qt", you will already be familiar with Kalle's style. Clear, concise and easy on the eye.
This book is the perfect companion to it. The difference being though that this is an answers book. You need to be familiar with using moc to compile some of the code example.
The uberfurer of dire programming books (Schildt) has something on the front of his books which goes along the lines of if you want answers quickly, just ask the expert. Unlike Schildt though, this is one book I would dip into.
It quickly and clearly explains how to do certain things in Qt (such as circular widgets - it really is an answers book, but it still teaches the reader how it works and how to best approach a problem. Matthias (and his co-authors) really do know what they are on about.
There is only one thing wrong with the book: it is not big enough. I hope the authors bring out a volume 2 (and 3) and possibly even a Qt4 companion.
I have been using Qt for quite a few years now and the material in there has made me look afresh at some of the practices I had adopted - it is an eye-opener.