UML is something that you need to learn by actually using it on a real project. This book provides the other half of what you need to learn it that way by describing what all of the different diagrams are used for.
This book starts out by providing some background on what UML is and why you should use it before moving on to cover step by step what all of the different modelling diagrams mean and what they are used for. In so far as telling you how to create the various modelling diagrams, what to include on the diagrams, and when to use the different diagrams this book lives up to its 'Pragmatic Introduction' description.
That said you can't just learn UML by reading this or any other book. The only real way to learn modelling techniques properly is by using them to model a real project that you are working on. The book doesn't supply any significant real world examples for you to work through (its not that type of book) so you'll need to have your own project to work with if you really want to learn UML. The project will determine what it is that you need to model and then this book will really become useful with helping you to work out how best to model your project. Once you have worked your way through the book for your first few projects producing the UML needed to properly model them you should then have sufficient knowledge of UML to be able to do most of the modelling that you require and only need to refer back to the book when you need to do something that you haven't needed to cover in prior projects.
The book covers the complete UML language so that experienced designers using UML will also find this a useful reference when they come across the need to use one of the less common constructs.