If you're looking for a book about the Arduino that is stunning value for money, just head out to your local newsagents and track down a copy of the latest Teach In from Everyday and Practical Electronics (or EPE). It provides an excellent introduction to the Arduino device and then, as a bonus, adds a bunch of chapters about PIC development and some other good stuff.
Like all of the EPE publications, this is well written, technically accurate and laid out in an easy to read manner. And you even get a CD-ROM with lots of useful stuff on it too.
Full Disclosure: Many years ago I helped Ian and Tony to write a Teach-In for the magazine. It's nice to see that Ian is still writing for them, there's a lovely piece from him about state machines in the back of this very publication.
A must-buy in my opinion.