Jane Feuer's "The Hollywood Musical" and Rick Altman's "The American Film Musical" are good places to start. Though I like both "Moulin Rouge" and "Chicago", I would suggest that there have been a lot of interesting experiments with the boundaries of the genre in the post-Studio era ("Umbrellas of Cherbourg", "At Long Last Love", "Pennies from Heaven", "Darling Lili", "Bugsy Malone", "The Eighties", "New York, New York", Lisztomania", "Phantom of the Paradise", "Velvet Goldmine")...
Robert Hunt