Jamey Aebersold has released 100+ volumes of his famous play-a-long series, and in my opinion, this is the most useful of them all. It is not meant for the beginning jazz pianist, but any intermediate to advanced jazz musician will find the material well worth his/her time. "Rhythm changes" refers to the chord progression occuring in George Gershwin's composition "I Got Rhythm". Rhythm changes fascinate jazz musicians for decades - it is one of the most used progressions in jazz tunes ("Anthropology", "Moose the Mooche", "Rhythm a-Ning", "Cotton Tail" etc just to name a few), it's incredibly fun to improvise over, and it remains very challenging for jazz musicians to this day. This book is the most comprehensive guide to tackling rhythm changes I've ever seen.
The book starts off by explaining the original rhythm changes vs. "jazz rhythm changes". It then goes on to dissect how to improvise in each section of the tune, in detail, from all possible angles. That's the most interesting thing about rhythm changes - there are infinite ways to improvise over it. This book provides a solid framework to understand the various popular devices a musician uses when approaching rhythm changes. Blues, whole tone, diminished scales, pentatonic scales, key centers, motifs, turnaround licks, etc... all organized systematically and explained with playing examples.
The next section deals with actually playing the tune - in all keys, in various styles. This is the best part about the book: rhythm changes can indeed be played in any musical style, at any tempo, any key, and Vol. 47 presents several well-written heads that well illustrates the range of possibilities one can achieve by mastering these changes. My particular favorite is the Gb example in bossa-nova style - pretty melody over a sweet groove! The accompanying CD provides backing tracks for all playing examples as well, and I highly recommend using them when learning various parts of the book.