IIRC Ambrosia's first album (c. 1973?) also folds out into a pyramid.
I think "Yessongs" is the most-foldiest album cover I own (like a triple gatefold and a colour booklet too.)
"Chicago IV: Live at Carnegie Hall" is a tremendously classy package (comes in a box with many foldouts & posters) -- too bad the album is so shitty ("greatest hits played slower with too much echo.")