The biggest seller is cookbooks and the second is diet books — how not to eat what you've just learned how to cook.

Rooney’s line is a characteristically wry observation about modern consumer culture and self-contradiction. By pairing “cookbooks” with “diet books,” he points to a cycle of desire and restraint: we buy guidance to make pleasurable food, then buy guidance to resist eating it. The joke lands because it exposes how industries can profit from opposing impulses—indulgence and guilt—while individuals oscillate between them. Beneath the humor is a mild critique of fads and the commodification of everyday anxieties, suggesting that “solutions” are often marketed to problems that other products have helped create.