I solved that with my deeper bookcases by making little stands of wood. I measured boards as long as the interior of the shelf unit and as wide as a paperback book. As tall as a standard hard copy. Then I placed the little "shelf" behind my hard copy books and place paperbacks on them so I have two rows of books on one shelf, the back row elevated above the front. Only works with deep bookshelves, tho. And if the shelf isn't high enough to house a double decker set of books, then just rise the back row high enough to be able to see what they are or stack them on their sides.