Remove all books from the bookcase.
Rock the bookshelf slightly to determine where it's uneven.
Push a wooden shim underneath the bookshelf, tapered end first. Press the shim in until the bookshelf stops rocking
Pencil a line across the shim where it touches the bookshelf. Mark where the shim was on the bookshelf as well, then remove the shim.
Cut along the penciled line on the wooden shim, using the utility knife.
Turn the bookshelf over on its side.
Press the wooden shim to the bookshelf edge, holding it together until the glue sets.
Allow the glue to cure according to directions on the packaging. After the glue is cured, the bookshelf is ready to be used.