I have read all the above books in one way or the other, and I think all the four books are distinct in their appeal and value. The books basically reflect the personality and thinking of the author(s) themselves. Here is what I think about the books -
Clojure Programming - Chas Emerick, Brian Carper, and Christophe Grand, April 2012
This one takes a different tack than the others. It is a thorough introduction to the language itself (though it does cover more advanced topics like macros, protocols, types and records, etc), but it also has 4-5 "practicum" chapters that go through practical tasks like working with RDBMS and CouchDB from Clojure, web development using Ring, Compojure, Enlive, and Amazon Web Services, testing, design patterns, and how you should set up your Clojure projects (using Leiningen or Maven). It's also a little different in that comparisons between Clojure and Java/Python/Ruby are scattered throughout (which was really helpful to me coming from Python). It's really well-written; one of the few books that is enjoyable to read, rather than slog through.
Programming Clojure - Stuart Halloway & Aaron Bedra, March 2012
A very concise and succinct book written primarily for people with Java, Ruby backgrounds. Covers all the basics and rationale clearly. It also manages to drive home the whole point of Functional Programming, Abstractions, etc. Being short, it doesn't cover some deep stuff like macros in detail (which is OK) and leaves a lot of things for the reader to explore and learn independently. A great introductory book overall.
Clojure in Action - Amit Rathore, November 2011
A nice book which focuses a bit more on the practical uses of Clojure. Covers a lot of interesting material about designing applications of massive scale with Clojure. It also covers some ground about day-to-day practices like debugging, profiling, IDEs, etc. An interesting book which you might want to treat as a reference manual.
The Joy of Clojure - Michael Fogus and Chris Houser, April 2011
Now this book is going to be epic. This book is for people who want to internalize the way of Clojure. It will tell you why some things are the way they are in Clojure and why that really matters. It's a lot like On Lisp. It shows the most idiomatic way of writing Clojure code. The focus is more on fun than the mundane enterprisy things that we usually do with Java. This is one book which every passionate Clojure developer should read & understand. After reading this book you will be able to appreciate Clojure much more than before.
Practical Clojure - Luke Hart, December 2009
Another short, and nice book which has updated material. It covers the Clojure v1.2 features like Datatypes & Protocols and is written in the same lucid and approachable way like Programming Clojure. It's a very welcome addition to one's Clojure bookshelf. There is some interesting material about performance, protocols, parallel programming etc. It has nice code examples, diagrams etc. If someone wants to learn Clojure today, this one might be the right book to choose.
Having said that, it all depends on what you are looking for. If you want to just learn Clojure, then pick Clojure Programming or Programming Clojure (yeah, the names are confusing! The former is from O'Reilly, the latter is from Pragmatic). If you already know some Clojure and want to use it in real life scenarios, pick either Clojure Programming (which has 4-5 chapters dedicated to "practicums" around database and web development, testing, etc) or Clojure in Action. If you really want to take the red pill & dive deep into the Lisp/Clojure rabbit hole, by all means choose The Joy of Clojure.
But no matter what, don't stop having fun with Clojure.
