Quantity

 There is precise subject category, name of author, musical settings, like "García Lorca, Federico, 1898-1936 Musical settings." For Lorca the number of items in WorldCat is more than 800, for William Blake, 1475, for Shakespeare, 5,000, etc... Dickinson, 1,600, Whitman, 1700.  Baudelaire, 500. This is imprecise, but it gives a rough idea. There will be duplications, and this counts both scores and recordings in libraries across the world, with inevitable duplication of results, with the same thing being counted more than once. I can't believe there are only 2 for Rilke! Celan has 200 or so.  Just for a ballpark, the category "Song Cycles" has almost 25,000 entries, and songs with piano 300,000. 

What this means: any treatment of the subject can only deal with a small percentage of the material. A catalogue alone would be a 500 page book, so a 300 word book about the subject matter can only deal with so much.