Lightroom 3 - The Complete Guide for Photographers by Martin Evening is one of the best books I've seen in regards to Lightroom 3. I first purchased his Lightroom 2 book a couple of years ago and when I upgraded to Lightroom 3 it was a no brainer for me to pick up the newest release.
One of the things I look for in a reference book such as this in addition to the actual content is the layout, the examples, the break down in steps in regards to a procedure. I was very impressed with the book's included screen caps and explanations. You'd be surprised how many books out there skimp on screen caps which I feel is an important part of a reference book such as this one.
There are a ton of Lightroom 3 books out there and I'm sure everyone has their favorites so I thought I would at least share mine as well.
Where To Buy: Find the lowest price on this book in the TPF Amazon Store located here: Lightroom 3 - The Complete Guide for Photographers