A paper cover over a hardcover (and occasionally paperback) book. Initially these were used to protect the hardcover binding from soiling and scuffing that would discourage sales. In the 1830s publishers started to see some advantage of printing titles and even advertising on these.
For a publisher like Grosset & Dunlap, the space on the front and back flaps and back panel of the dust jacket were valuable space to promote the present series as well as others that readers might like to buy. Thus, a girls’ mystery would routinely advertise other girls’ mystery series from the publisher.
During certain periods, especially from the mid-1920s to the early-1930s, publishers had more comprehensive lists of the series offered on the inner surface of the dust jacket.
Dust jackets are pretty to look at because they often contain the only color illustration for a book. However, they are also valuable evidence of the approximate printing/binding era for a copy in hand. Assuming that the book and jacket have always been together (see “mismatch”), the series volumes that are advertised usually represent all of the titles available at the time of printing. Looking at the last title listed for each series and looking up the copyright date for each, one can determine that a jacket is no earlier than the most recent of these dates.
The books were printed in larger numbers than the jackets. Thus the dust jackets changed more frequently. They are a better guide to the approximate printing/binding date than the “post-text ads” or “pre-text lists” in the books are.
Dust jackets represent a significant portion of the value of a collectible book. It is common for a copy in a dust jacket to sell for several to many times the price of a book without a jacket.
Part of the reason for this is the aesthetic desirability of the illustrated dust jacket. Another part has to do with the more accurate dating possible when the correct jacket is still present. Another is the fact that these paper wrappers are fragile and ephemeral. They are easily damaged or thrown away.
It is quite common for a copy in DJ to sell for two or three times the value of the same condition book lacking the DJ. In these cases, 50% to 66% of the potential value is tied up in the DJ in collectible condition.
In an extreme example, the original first printing of Tarzan of the Apes published by A.C. McClurg in 1914 (not an A.L. Burt or Grosset & Dunlap reprint) can be worth $1,000.00 without a DJ or $50,000.00 with a very good condition DJ. In this case, 98% of the potential value is associated with the dust jacket.
Because of this, and other reasons associated with circulation, library supply houses make protectors for dust jackets. These are used by libraries and collectors and booksellers. The best kind have a paper backing and a non-acid plastic outer cover. The Demco PaperFold line is popular with many booksellers and collectors. Other brands are Gaylord and BroDart. Each company has several grades and these should be compared carefully to reach the desired range of protection vs. economy.