Between 1954 and 1993, Mad released 220 paperbacks books. Ninety-four contained previously published material. A history of their publication can be found here.
When I was around 11 or 12, the K-Mart a mile from my house (which was closed a few years ago) had an impressive collection of them, and I had to have them all (I have 40). I realize now this was an early sign of obsessive-compulsive tendencies (this has manifested today in many ways, one being my need to read and own every single one of John Sanford's Prey series). As soon as I'd accumulated enough lose change or allowance money, or both, I'd get on my bike and make that ride to K-Mart to get my fix.
I generally chose them based on cover, and here are some of my favorites:
The back covers were usually demented, as well. You've got to love the use of the word "clod". That's a word we should all work on bringing back.