Older solid-core wooden doors have a lot of character and it can be difficult to just toss them away if you're remodeling or come across one cheaply at a building thrift store. Better Homes and Gardens has a great tutorial on how you can transform that door into a useful and attractive corner shelving unit.
What you basically need to do is mark the centre point of the door, drawing a vertical line along that point, and cutting the door in half vertically. Sand the cut edges and pre-drill pilot holes to join the two sides together with timber screws. For the shelves you'll want to cut out quarter-circles and pre-drill and screw those shelves into the door. Now you just need to decide if you're going to paint or refinish the door.
One safety concern to keep in mind if you attempt this project is that many older doors may have been painted with lead-based colours -- if you're unsure wear a respirator to avoid breathing in potential lead particles while cutting, sanding, and refinishing. See the source link below for precise measurements and additional photos.
How to make shelves (using an old door) [Better Homes and Gardens]