A bookend that fits 5 books. There are two stl files, one for smaller books and the other for bigger/heavier books. The one for bigger books just has a bigger base to prevent it from tipping over. 

I recommend printing with painted on organic supports to prevent the print from moving around with the extruder as it usually happens when printing thin tall objects.

/Edit, added stls so extra book slots can be added. Its 4 extra slots per module.