Pick a corner of the sticker away with a thumbnail and peel the sticker back slowly. If you can feel the sticker lifting paper from the book, stop and soften the glue before continuing.
Warm the sticker with a hairdryer at a low setting for a minute and try to peel it off. Repeat three or four times, if the sticker doesn't come away immediately. Heat softens many types of glue sufficiently to remove the sticker.
Soak a cotton ball in warm water with a touch of dishwashing liquid. Blot the sticker until it is damp and gently try to lift or scrape the sticker away with your nails. Many glues are water soluble. Pat the area dry with a paper towel immediately afterward, so the moisture doesn't warp the book's cover.
Soak a cotton ball in mineral spirit and dampen the sticker, if neither heat nor water softened the glue, before gently peeling off. Mineral or white spirit is a solvent for most price label glues, and so is lighter fluid.