According to Joseph Scoleri's "Ace of Aces Collector's Guide" (http://maverick.brainiac.com/aoa/aoa1.html#08):
"...the paper cover editions were described by Nova designer Dennis Greci as "an unfortunate attempt to produce books less expensively." Instead of being packaged in a slipcase, this edition was wrapped in two sleeves made of stiff paper. This was shrinkwrapped with the front and back slipcase artwork displayed inside the shrinkwrap.
The gamebooks no longer featured the textured leather look of previous editions. Instead, the covers were made of the same stiff paper used for the outer sleeve. Greci passed along a word of warning for owners of the paper cover editions: "Unfortunately, the covers don't hold up well to continued use. They are actually dangerous to inanimate wet objects . . . ink tends to bleed out of the covers when they become wet."