So what do I think Adobe Photoshop Lightroom 2? Let me preface it by saying that I loved the first version, but I think that this version is much better. Why? Two reasons that I can pinpoint directly.
First, in the 17 months since the first book was released, the author has been training Lightroom and putting on workshops. I believe that he has learned not only how better to work with Lightroom, but how better to answer the kind of questions that his workshop participants have with regard to the use of Lightroom. Second, which may also come indirectly from the first, is that while much of the content is reused from the first book, the layout is much more concise and has a much more logical order, and therefore easier to understand.
Certainly if you do not have the first version then you should by all means get Adobe Photoshop Lightroom 2, but what about if you have the first version? I think that depends on your situation and your finances. If you have the first version and have been using Lightroom for the last year or so, you can probably figure out much of the new version, but so much has changed in Lightroom since version 1.0 on which the prior version was based, I would seriously recommend upgrading to Adobe Photoshop Lightroom 2 Book For Digital Photographers as you might just learn a few new tricks. I very highly recommend this book.