If you want to play jazz, you must have this book. It's just that simple. If you only have money for one book, this should be it. To take advantage of this book, you must be able to read bass clef and you must be able to understand chord notation.
If you go to any jazz open jam, you'll see people these real books. The horn players will have the Eb or Bb ones, piano guys will have the C one, singers will have the one with the lyrics on it. Once on the bandstand, you will hear someone saying "Let's play the tune XYZ on page 123". You will open this book and glance through the changes. With some study, you will know what to do, even if you never saw the tune before.
But to do this you'll need this book. To improvise on any tunes you will need to know the melody from this book. It is also great for practise reading.
You can recognize the guys that practise a lot when you see their real books are falling apart.
I saw some people with the xerox copy of this. Call me naive, but if you consider how many authors spent their lifetime writing these songs, it is well worth the thirty bucks or so that Amazon charges for it. Also, I get paid to play these tunes. Why shouldn't the authors or their estates get paid as well? At least this is how I feel.
This is a must have.