Finally, I found in "The American Film Musical" a critic who takes this genre seriously and has worked to understand its intricate and complex layers and patterns. Altman divides his study into three "subgenres" - the fairy tale musical, the show musical, and the folk musical - and demonstrates how most every example of the genre falls into one of these categories to some extent.
As well as being devoted to the musical, Altman also provides a method for genre study in general, which some readers will want to skim or even skip if their only interest is research on the musical. However, to truly understand the workings of the genre it is beneficial to read the whole book, because the musical is surely one of the most complex art forms ever, and Altman does so much to break into the problems that many critics have ignored that it's worth being enlightened as to his process.
The book makes you reconsider all the presuppositions you may have about the film musical and what constitutes a musical. The patterns and workings he reveals, particularly with regard to the dual-focus narrative, are fascinating: instead of just the old "how many songs?" question, Altman uses the criteria of "coupling" to form his theory of what constitues a musical.
A definite YES for anyone searching through the dark for a more modern study of the genre.