A fragile copy of The Penguin Thelwell, circa 1963, featuring frowning little girls and their grumpy and stubborn ponies. It's possible some of these cartoons remind me of my childhood. (Which did not occur in 1963, thank you very much.)
My favorite,Vanity Fair (the Thackeray novel, not the glossy rag). I reread this about once a year and misplaced my old copy, so picked this one up for $2 at my favorite used bookstore the other day. Cheaper than the magazine, and Becky Sharp is so much more interesting than current cover girl Kate Moss.
Incidentally, Penguin is merging with Random House. I am hoping they will call the new entity Penguin House.