An interesting write-up on Dust Jackets can be found in Wikipedia,
following is an exerpt:
After 1900, fashions in, and the economics
of, publishingcaused book bindings to become less decorative,
and it was cheaper for publishers to make the jackets more attractive.
By around 1920, most of the artwork and decoration had migrated
from the binding to the dust jacket, and jackets were routinely
printed with multiple colors, extensive advertising and blurbs;
even the underside of the jacket was now sometimes used for
advertising.
As dust jackets became more attractive than
the bindings, more people began to keep the jackets on their
books, at least until they became soiled, torn, or worn out.
One bit of evidence that indicates when jackets became saved
objects is the movement of the printed price from the spine
of the jacket to a corner of one of the flaps. This also occurred
in the 1910s and early 1920s. When jackets were routinely discarded
at point of purchase, it didn't matter where the price was printed
(and many early jackets were not printed with any price), but
now if book buyers of the 1910s and 1920s wanted to save the
jacket and give a book as a gift, they could clip off the price
without ruining the jacket.
Prices for dust jackets have become so inflated in recent years
that even early reprints of certain titles in jacket can command
good prices.