These guys were really hit and miss. I still manage to find a few of their paperbacks (usually the later, more modern authors) cheap now and then but very rarely find I'm impressed. The last Zebra I picked up was Carnivores by Penelope Banka Kreps (posted an article on it here - 031183.blogspot.com/2011/01/carnivores-by-penelope-banka-kreps.html).
Really dreadful garbage and hardly worth the $2 I blew on it.
Some of the Zebra covers though, well, they'd be worth ten times that alone. Actually, in a lot of cases, a Zebra book minus the book would be worth more money...
More Zebra paperbacks to add: Night Bait and Night Lust , both from 1982, both by Philip Straker (actually an Ed / Edward Lee pseudonym). Artwork by the same person who did Act of Love by Lansdale and The Sanction by Johnstone...obviously has a fruit fetish.