Do YOU know that Post-it notes leave a residue of adhesive when you remove the note? These "self-stickum" notes are being used on books, periodicals, documents, etc., etc., etc.
PLEASE think before you use them, and do not use them on valuable materials, or even on—or as book markers in—reference materials.
There will be a reckoning! Dirt will adhere, pages of books may stick together, and books may stick to their neighbors on the shelf.
Post-it notes are a great convenience but—as with Scotch-tape—we will wish we had not used them when the damage they cause begins to show up!