Designed by Dang Stowe and featured in How to Build Bookcases and Bookshelves (edited by Scotts Francis). Can hold a number of books and slide along the rails to brace them well without the risk of the movable brace sliding away. Since it isn't my design I won't go into a lot of detail on the project.
What you will need
Red Oak or your choice of wood
How to do this project
Pretty straight forward. Two tall braces cut to match and a smaller end piece that anchors the rails that the books will sit on. One brace and the end piece are glued so as to be fixed in place whereas the remaining brace is able to slide along the rails and support however many books you choose to display. The top left corner (or anywhere else on the braces you want to put them) hosts a purely decorative design.