Another fun one from Confessions of a Bookplate Junkie. The text reads:
who folde a leafe downe ye divel toste brown
who makes marke or blotte ye divel roaste hot
who stealeth this boke ye divel shall cooke
UPDATE: Check out this page for some ancient Assyrian book curses. (Technically they were clay tablets, but the sentiment transcends the form.) I like this very thorough warning:
He who breaks [this book] or puts it in water or mbs [?] it until you cannot recognize it [and] cannot make it be understood, may Ashur, Sin, Shamash, Adad and Ishtar, Bel, Nergal, Ishlar of Nineveh, Ishtar of Arbela, Ishtar of Bit Kidmurri, the gods of heaven and earth and the gods of Assyria, may all these curse him with a curse which cannot be relieved, terrible and merciless, as long as he lives, may they let his name, his seed, be carried off from the land, may they put his flesh in a dog’s mouth.