A slipcase is a hard cover like an open box that a book slides into. The spine of the book usually stays visible; the other three sides are protected. Sets, special editions, and gift books often come this way. You slip the book in and out — that is the whole job of the thing. It is not the paper jacket wrapped around a single book (that is a dust jacket), and it is not a bag. Bookshops, collectors, and publishers use the word; everyday speakers may just say “the box the book comes in.” Neutral, slightly specialist.