Got a book of these recently, here are are some of my personal favorites.

Lord Sandwich: “Really, Mr. Wilkes, I don't know whether you'll die on the gallows or of the pox.” John Wilkes: “That will depend, my lord, on whether I embrace your principles or your mistress.”

Bessie Braddock: “Mr. Churchill, you are drunk!” Winston Churchill: “Yes, Madam, and you are ugly. But in the morning I will be sober.”

Nancy Astor: “Winston, if you were my husband, I'd put poison in your coffee.” Winston Churchill: “Nancy, if you were my wife, I'd drink it.”

Young man: “I can't bear fools.” Dorothy Parker: “Apparently your mother could.”

“That's not writing, that's typing.” -Tom Stoppard on James Joyce EDIT: Truman Capote on Jack Kerouac (thanks jleonardbc)

“He had a mind so fine that no idea could violate it.” – T.S. Eliot on Henry James

“He has the lucidity which is the by-product of a fundamentally sterile mind.” -Aneurin Bevan on Neville Chamberlin

“His mind was like a soup dish, wide and shallow; it could hold a small amount of nearly anything, but the slightest jarring spilled the soup into somebody's lap.” – Irving Stone on William Jennings Bryan
link by