I recommend reading Scott Miller's guides to musical theatre, they are composed of thorough analyses for different musical plays and sheds new light to some musicals that are considered dated by today standards, such as Oklahoma!, My Fair Lady or The Cradle Will Rock (all three amazing musicals, in my opinion):
- From Assassins to West Side Story - analyzes Assassins, Cabaret, Carousel, Godspell, Gypsy, How to Succeed, Into the Woods, Jesus Christ Superstar, Man of la Mancha, Merrily We Roll Along, Les Misérables, My Fair Lady, Pippin, Sweeney Todd and West Side Story
- Deconstructing Harold Hill - follows Miller's first book, this one analyzes Camelot, Chicago, The King and I, March of the Falsettos, The Music Man, Passion, Ragtime and Sunday in the Park with George
- Rebels Without Applause - analyzes groundbreaking musicals such as The Cradle Will Rock, Pal Joey, Oklahoma!, Anyone Can Whistle, Hair, Jacques Brel, Songs for a New World, Floyd Collins and Rent
- Sex, Drugs, Rock & Roll and Musicals - analyzes rock musicals such as Lippa's The Wild Party, Grease, The Rocky Horror Show, The Best Little Whorehouse in Texas, Bat Boy and Hedwig and the Angry Inch)
I also recommend Joseph P. Swain's The Broadway Musical, which analyzes the way a musical achieves plot, conflict and character development by the use of music of different musical plays (Show Boat, Porgy and Bess, Oklahoma!, Carousel, Kiss Me Kate, The Most Happy Fella, My Fair Lady, Camelot, West Side Story, Fiddler on the Roof, Godspell, Jesus Christ Superstar, Evita, A Chorus Line, Sweeney Todd and Les Misérables)
Ethan Mordden's series on how the musical theatre art form evolved through the decades is also a must-read