I needed to fit some shelving into a space that was too small for standard sized shelves and I could not be bothered making shelves from scratch so decided to use IKEA Expedit bookcases and modify them to fit the space.
The Bookcases were about 350mm to wide for the space I was filling so I cut them down to size. I also wanted one of the units to be suitable for holding CDs so I added some extra shelves on one of the bookcases using leftovers from the other two.
I am not going to show all the assembly steps. The key thing here is shortening the thick outside frame of the bookcase.