I bought the eBook version of this book from the publisher's website about two months before the official release because I needed a good clustering reference. As soon as it was released, I had my boss buy the printed version for our team to use at work.
Failover clustering is very critical to our high availability strategy, and clustering experts are in short supply. I don't expect it to make me an expert on the topic, but so far it has served me very well as my main resource for in-depth information on clustering.
I've recommended this book many times to others. I have found the information in some of the other books to be flawed and not always correct. This book is not only authoritative on the subject, but it has proven to be more accurate than the other books I've checked out or heard about.