A slipcase allows for the spine of a book(s) to be visible. It is great way to store a book that is in stable condition or in a mylar wrapper due to the repeated abrasion of pulling a book in/out of a slipcase. They can hold one or more of the same sized books. They can have multiple dividers or one large space. They can be very plain and simple, letting the book spine speak for itself or they can be a work of art in itself!
