Rectangular enclosures that are open on one edge for the insertion of a book. The open edge may either be at the top (top-loading slip cases) or on one side (side-loading slip cases). The earliest English examples of side-loading slip cases date from the last quarter of the eighteenth century and have flexible leather sides (collapsible side-loading slip-cases). Entirely rigid, leather-covered top loading examples (rigid top-loading slip-cases) were used for fancy pocket books and almanacs from at least the mid-eighteenth century and, covered in printed and decorated paper, were widely used for such books in the early nineteenth century. They were also commonly used for folding maps.
Roberts, Matt, Don Etherington, and Margaret R. Brown. 1982. Bookbinding and the conservation of books: a dictionary of descriptive terminology. Washington: Library of Congress.