One big difference between books and sheet music: nowadays, any reasonably experienced musician can identify the publisher of a piece of sheet music from about twenty feet away. The pale-green-plus-giant-composer-name design of Peters; the yellowish-beige-sans-serif-small-caps of G. Schirmer; the flush-right-lower-case-bold-black-and-white of Universal Edition, etc., etc. Even a Dover score, with its clip-art-plus-white-text aesthetic, can usually be spotted on a podium stand from the second balcony.
Which is why I still love hunting down antique scores. As my lovely wife will confirm with a roll of her eyes, I love old stuff, and that includes sheet music. Popular sheet music, of course, is extremely collectible on account of eye-catching covers, but there was a time when classical wasn't that far behind. Here's five favorites I pulled from my shelves.
Monteverdi/Krenek: L'Incoronazione di Poppea (Universal Edition, 1937). Pure neo-classicism, by way of Napoleonic archeological surveys. I picked this up at a Boston Conservatory library sale; from the library of Ingrid Kahrstedt Brainard, the early dance historian, which is pretty cool.
Clementi/Tausig: Gradus ad Parnassum, ed. Gustav Damm (Steingräber-Verlag, n.d.). No date, but I'm guessing sometime before absinthe became illegal. I can imagine Alma Mahler wallpapering her bathroom with that pattern.
Joseph Marx: Marienlied (Universal Edition, 1925). The lithograph is by Alfred Keller. I don't know what Rubenesque nudes holding up a proscenium arch in front of craggy landscape has to do with the song, and I don't care.
Penderecki: Quartetto per archi (Polskie Wydawnictwo Muzyczne, 1963). A classic piece of abstract expressionism from the Polish state music publisher, who opted for these sorts of far-out covers quite a bit. Purchased at a used-book sale in Chicago many years ago—the previous owner was composer Alan Stout, which I initially thought to be an amazing coincidence, until I realized that there were probably at most a couple dozen copies of this in all of Chicago.
Janáček: The Cunning Little Vixen (Universal Edition, 1924 [reprint]). This is such a fantastic cover that it's criminal there's no artist credit. (And I looked—it does seem to be the same artist who did the cover for UE's edition of Krenek's Jonny Spielt Auf). It's been years and years since I saw an album cover or movie poster that good. In some ways (not many, but some), the old days were better.