Got this 2nd hand. ElasticSearch has changed significantly so you'll be tripped up by this (e.g. dynamic scripts are disabled by default). Further, the teaching is dire, it's pours stuff at you with no filtering for relevance (e.g. page 26 mentions Lucene term vectors without context), and describes stuff ambiguously. Not good for the beginner it's supposedly aimed at. If you can get it cheap and use google maybe it's ok. Further, packt seems to require that you have an account on their site (requiring your name + email) to download the code printed in the book, so I'm not buying Packt books again. I can't imagine how the ox-witted executive who imposed that policy thought that a good idea.
In a nutshell: If you are experimenting with or considering using ElasticSearch in production then I would strongly recommend that you buy this book. As mentioned by another reviewer, there are plenty of bad (and out of date) ElasticSearch tutorials floating around the web, but this single book presents a well-paced and logical introduction to ElasticSearch, and also an excellent guide of how to configure it and use it within a production deployment.
The book begins by discussing the basic search concepts and how ElasticSearch implements them. As the book progresses so does the level of complexity, but it is easy to pick and choose the information required. The chapters on the advanced topics are worth the price of the book alone, and it is hard to find the same quality of coverage on the web in relation to topics such as percolation, troubleshooting or administrating a cluster.
One of the authors, Rafal Kuc, is an authority in full-text searching and indexing, and he has written several successful books on Solr (the other popular open source search engine), which have been very useful to me as I have used search engines more and more over the past year in my daily work as a software developer. Rafal has a gift for providing exactly what a developer requires in order to understand the key concepts of search, and also provides excellent instruction in how to implement this effectively in the real world.
In summary, this book comes highly recommended for learning about the up and coming ElasticSearch!