Hey, DR Elmore, how is the musical theater book? Worth a look?
Evry book has something to offer, but almost all of them fall down on matters concerning the turn-of-the-century (19th-20th) materials because it's too distant and they know too little. From OKLAHOMA! on, the musical is very well documented on disc, film, paper, etc., but there is litle to no documentation beyond newspaper clippings and memoirs for such early forms like the minstrel show, the medicine show, or the extravaganza. Creators of "The Golden Age" were old enough to have seen an actual minstrel show, Lillian Russell and other legendary stars like Mabel Barrison and Fred Stone (Oscar Hammerstein I produced "Naughty Marietta" and Dillingham, who produced the 1905 "The Red Mill" produced into the late 20s), but no one today has. Stephen Sondheim showed with "Please Hello" that he can parody Gilbert and Sullivan a bit, but Gershwin, Weill, Porter, Rodgers & Hart all wanted to write a G&S show. Some did it well: OF THEE I SING, KNICKERBOCKER HOLIDAY, DEAREST ENEMY and they're some of my favorites.
There was a period where classic operetta revivals could successfully have a decent run but today the smartass kids on the internet would kill their chances. They can be merciless with some of the Encores! productions for being "old-fashioned" or "bad construction," and the goal there has always been to present material that may not succeed in a legit revival or to examine the works of an earlier period for what it was. Audiences probably have always had the smartass audience percentage - I was certainly one once - but their closed minds, superior attitudes and bitchery can certainly kill a lot of fun in being an audience member.
I find it all fascinating, but few of the researchers writing about "musical theatre" want to even deal with such old-fashioned materials like operetta and revue, even though those forms - somewhat mutated - exist today as part of the American musical theatre.