Who knew there was such a market for rejected New Yorker cartoons? The Rejection Collection: Cartoons You Never Saw, and Never Will See, in The New Yorker (which we noted upon its release) apparently did well enough to spawn a sequel: The Rejection Collection Vol. 2: The Cream of the Crap.
Those who really go in for cartoons that never saw the light of day may also appreciate Killed Cartoons: Casualties from the War on Free Expression, a collection of editorial cartoons that got spiked from various newspapers for various reasons.
Finally, if we may leap to cartoons that were no doubt jettisoned from generations of classrooms, a massive two volume set collecting the complete cartoons of Mad Magazine legend Don Martin. Hard to go wrong with that.