3M Scotch® Adhesive Remover Pen

The secret weapon of teachers, librarians, and obsessives.

From the website:

Return every book to mint condition with the Scotch® adhesive remover pen.

The pH-neutral pen is perfect for removing all types of adhesive overspills, leaving books crisp and neat.

Remove unsightly graffiti from desks, lockers, walls or glass, and lift grime and grease to make the busiest work sites sparkle.

It's photo–safe and easy to use, so you can work quickly and confidently.

With just a touch, every book and surface will look its best.

Citrus–based.

$3.49 here.