The dust jacket (sometimes book jacket, dust wrapper or dust cover) of a book is the detachable outer cover, usually made of paper and printed with text and illustrations. Read more about the history of the dust jacket here.
"Some of them are worth far more than the books they cover. The most famous example is the jacket on the first edition of The Great Gatsby by F. Scott Fitzgerald, published in 1925. Without jacket, the book brings $1,000 or so. With the jacket it can bring $20,000 or $30,000 or more, depending on condition. One copy in a near mint jacket was listed for sale in 2009 for half a million dollars."
above image: a dust jacket and book printed by bigger dot for zova books