A few years back I purchased a copy of Poems from the Hobbit, which had a sticker covering something like a third of the entire cover.

It removed all right with a hair dryer though. But due to the size of the book the sticker was even more absurd than usual.

My most recent ex-wife used to work at a used book store. They used small amounts of lighter fluid on a cotton cloth to loosen really stubborn stickers. It doesn't take much, but I've had some success with it. (I don't own a hair dryer so that isn't an option in my house.)