A Fanny Full Of Soap looks promising. My Amazon wish list was newly invigorated.
A book I recently came across that I read quite a bit of and plan to finish is The Theater Will Rock by Elizabeth Lara Wollman. It's about rock musicals and popular music's influence on the sound of musical theater and vice verse.
Mandelbaum's A Chorus Line and the Musicals of Michael Bennett is a must, yes. I am so fascinated by his aborted musical Scandal. I wish someone would make a movie about that story of how he came to shut it down.
I like Ethan Mordden's books all around--he is certainly biased, but I appreciate his point of view on some matters such as why there are musicals that attempt to redeem murderers (Ragtime, The Capeman) during the Clinton era. He makes a distinction saying that Assassins doesn't redeem them, but allows us to understand them. "The Happiest Corpse I've Ever Seen" is better than I thought it would be but not as gratifying as the earlier volumes. The research on The Wild Party isn't as illiminating as I'd hoped, but it was interesting reading a musical I saw on Broadway being put in historical context. He surrrrre loves his footnotes. And Side Show, natch. =)
With Irma you gotta do something!