The best book to buy right now for a beginner's look at DB2 is the Understanding DB2 book. The one that is available today is for V8.2, but it covers everything that you need to know about DB2 in a very easy to understand way. I like the way the authors added a mini case study at the end of each chapter to solidify the material covered in the chapter. There are also 5 or more study questions at the end of each chapter that can also help ensure that you've grasped the concepts.
The subtitle of the book is "Learning Visually with Examples" and the authors did a fine job of including many diagrams and screen captures throughout the book to make learning a bit more interesting. Check out the positive reader feedback that is posted on the amazon.com site linked above.
Also, I've had feedback from people who were trained on our databases or platforms (Oracle or DB2 for z/OS) and mentioned that they found it amazingly easy to transfer their skills to DB2 Linux, UNIX, Windows with the help of the book.
The newest version of this book comes out in the next month, Understanding DB2 Second Edition so you may want to wait till it is available, or you can buy the 8.2 version now at a reduced price. The best place for a European to buy this book is from Pearson Education EMEA Bookstore where this book is now available for £27.94; free shipping in Europe.
Another site worth checking is our IBM Information Management Bookstore where we have information about a deal where you can get 35% off all IBM Press books ordered before the end of December. Free shipping in the US.