Although my bailiwick is "classical" music -- and so, I have found the IMSLP to be an invaluable resource -- I have more than a passing interest in popular music, especially from what I call the Golden Age of American Popular Music, by which I mean popular music published and/or recorded in the U. S. during and preceding the 1930s. The vast majority of these songs and dances made it into print before 1923, the current cut-off date for PD. Because of the life + 50 years requirement in Canada, many post-1923 popular songs and dances that are still protected in the US are now PD in Canada, where IMSLP is hosted.
My father, Dr. Saul Starr -- who was a pediatrician -- amassed over his lifetime an immense collection of sheetmusic of American popular sheetmusic, easily over 100,000 items. It was then one of the largest collection in private hands, probably second only to that of his friend Lester Levy (whose collection is now at Johns Hopkins.) When my father died in the 1960s, his collection went to the Music Library at the University of Indiana in Bloomington.
Apart from the small portion of the Lester Levy Collection that is accesible online, most of the popular sheetmusic from this era is very little known -- even though the songs, through recordings and subsequent arrangements, are part of the everyday fabric of musical life in the US and the world. The importance of this sheetmusic is threefold. First, here are the original, undoctored versions of these songs and dances -- before countless performers and arrangers reworked them to display the personalities of particular singers and musicians. The first reason is self-evident, but the second is less well-known. The publication of popular music, especially in America, coincided with the technical development of color lithograthy, engraved printing and commercial photography. Most editions of classical music had (and still have) monochrome covers with very little artwork -- except perhaps for florid frames around the titles and composers' names. By contrast, editions of popular songs had spectacular cover art. The covers were often lithographed in an amazing profusion of colors. The artistic designs were sophisticated and imaginative. The wealth of decorative fonts was astounding. The images are often of considerable historical significance. In sum, the covers of early American popular music form a genre of graphic art all its own. Virtually all of this material is little-known to the public today, except for the relatively few pieces that have been reprinted in anthologies. Here is a treasure trove that represents what I think was the zenith in the cultural history of American popular music -- and very few people have ever seen it. The third reason is perhaps the most compelling. A huge portion of American popular music has disappeared from current musical life because the sheetmusic has remained inaccessible ever since it went out of print. The wealth of unknown musical treasures in this literature is stupefying.
What I think is called for is a project similar to the IMSLP for popular music in the public domain -- or an expansion of the IMSLP to include published popular music. There are now a few ragtimes and similar pieces in the Petrucci Library. But they represent only a drop in ocean of popular material that was published and is now PD.