Cover proofs are the wrappers of paperback books that have not been bound to a book. Some are sent to booksellers for marketing display, others included with uncorrected proofs sent to reviewers. More often than not a hole is punched in them by the publisher, and sometimes a corner is trimmed. Below is a selection from some of my favorite books.
Titus Groan by Mervyn Peake. UK proof covers are scarce. I have the set of three from this series.
Bridge of Birds by Barry Hughart. Notice the hole punched right on the price. This is typical.
Armageddon Blues by Daniel Keys Moran. This cover has extra information printed on the rear.
The Digging Leviathan by James P. Blaylock. This cover is missing the bar code information; apparently that is printed in a separate step.
Silverlock by John Myers Myers.
The Red Magician by Lisa Goldstein.
The Web Between the Worlds by Charles Sheffield.