J 

Jacket

 the cover.

Japon Vellum

 A smooth, glossy, durable paper that looks and feels a little like vellum but is made from native fibers and is produced in Japan. Most commonly found in fancy or editions de luxe.

Jobber

 A type of distributor who provides books to that works on a smaller scale than wholesalers and provides mass market titles to airports, grocery stores, drug stores, etc.

Joint

 The exterior juncture of the spine and boards of a (usually) case-bound book.

Journal

 A periodical or magazine, especially one published for a special group, learned society, or profession.

Justify

 Positioned lines of text so that the left and right margins are evenly set down the side of a page.

Juvenile

 A children's book.

Juvenile Picture Book Binding

 A sturdy style of binding that is designed for books used by children. The book jacket is usually a case.