I am sure there are lots of great resources for getting to grips with Ubuntu and Linux in general. However, I remember my first tentative steps were helped along by being able to read a manual. So having a reference work freely available for Ubuntu is great.
To be honest, I cant claim to have read this book, but I like the idea of giving the PDF away for free. Certainly more than a few people who read the PDF will be motivated to buy the printed copy. Check it out for yourselves at http://www.ubuntupocketguide.com/