Recently got hold of Trunk Records' book of library music lp covers from the 60s and 70s. Library music was (and still is) music created for use in film, TV, radio and adverts by various studios, and because it was never commercially available, the musicians could frequently be as experimental as they liked. The same goes for the artwork. There's some wonderful and bizarre stuff here - some of it's so bad it's good, and some of it's so bad it's just terrible, but there's some genuinely strong design there. There's a few examples here.
Not in the book but also on Trunk is the reissue of Basil Kirchin's Abstractions of the Industrial North from 1966. Great music, great cover.