I know not many here write musicals, but I have been reading an excellent book lately I want to recommend.
"Making Musicals" by Tom Jones - www.limelighteditions.com - Hal Lenoard
Just a delightful and very informative book by half of the team who wrote "The Fantasticks". A sort of how-to book, but more in general terms. Even if you do not write musicals, I think many a composer would find it an interesting read.
I have found some of his advice and tips to be quite useful to me as I write my own musical.
Perhaps others have book suggestions that would like to share as well.