This one is a must for Musical Theatre buffs and students. There are plenty of useful details about the history, background and biography of writers, composers and stars, and all those little bits of gossip that just help things along.
The book has the snowball effect as well: the further you get into it, the less you want to put it down and the more you want to know.
John Kenrick has a pleasant style, fairly easy to read in spite of the fact that things occasionally get pretty complicated. The book is about how it all happened, and how it keeps changing. It is for people who like their history fairly neat; it's not the book for those who want to discuss theatrical interpretation or a discussion of stars' playing style.
For the British it is lacking in detail from our perspective, though that would be better remedied by a British expert writing their own history of Musical Theatre. Some of Kenrick's remarks do come across as parochial, or even resentful of British style from the 80s, which is rather disappointing as the rest of the book is so well-balanced. You can't have everything.