As I was wandering through the literature section of the library that employs me, it occurred to me that the fashion for multi-volume Collected Works seems to have ended around 1920 or so, and to wonder why.
Specifically, the fashion was for large sets with volume numbers on them—the works of Herman Melville or James Whitcomb Riley or Robert Louis Stevenson, or Thomas Hardy or Charles Dickens, all in matching covers with volume numbers stamped on them. You can of course purchase the complete works of Kurt Vonnegut or Maya Angelou or Anthony Burgess, with matching fonts and spine layouts, and they will look lovely on your shelves and sometimes make a picture across all the spines, but they won’t have volume numbers on them.
The only exception, I think, is for playwrights. Faber, particularly, has titles such as Plays: Five.
I don’t have any idea why the volume number thing became so unfashionable. I suspect that it had to do with fashions in home design as much as about books themselves, but I don’t really know. If it were a trend for a modernist aesthetic, why did it stop as early as it did, rather than, post-WWII? And why would it have been difficult to incorporate volume numbers? It seems very odd to me.
Tolerabimus quod tolerare debemus,