Well, I'll admit to being a little biased, but in my opinion this is one of the finest pieces of theatre ever written. I also think it's the first time that Stephen Sondheim's music is not the main attraction. Thats doesn't mean the score isn't amazing - it is one of Sondheim's finest. But the direction, staging and book, all by James Lapine, are equally beautiful.
This book, therefore, is a must if you're hoping to get to know the play. Sitting in comfy chair with the original cast album (the London recording just doesn't measure up) of Passion blearing through headphones whilst reading this book is, I think, one of the most amazing musical/literary/dramatic experiences one can experience. The story is fascinating and addictive and the music haunting and intoxicating. However, You do need the recording AND the book to fully appreciate that. They really are not stand alone pieces but together - that is something special.
(If you're a musician - don't forget that Hal Leonard have just released a revised edition of the vocal selections for this show - that's worth getting too!)