I'm often asked where I find such great doors for projects. I'll share a little secret; they don't start out that way.
It's the ugliest door that has the most potential. Believe it or not, they usually start out like this simple single panel door. I prefer a door that has been painted several times over the years in different colors. If you want a lot of character look for peeling paint.
Start by scraping off the loose paint. Then sand the door feathering the scraped edges to prevent further chipping.
I use Fiddes Supreme Wax (my favorite color is Rugger Brown) to finish the door giving it an antiqued look highlighting the layers of paint, accentuating any naturally occurring crackling, and warming up an otherwise less than appealing color. If you're happy with the original color, use Fiddes Supreme Wax in Light which is actually clear. You'll get the soft finish without changing the color.
There are many types of old paneled doors. A four panel door like the gold one is a perfect candidate to be cut in half down the center for a corner shelf unit. Simply refinish the old door as described above.
Before putting the two halves together, I added the shelf supports to each side of the door while I had them lying side by side so they would be in perfect alignment.
Then, using a piece of square stock the thickness of the door, screw the two pieces together from the back.
Finally, starting from the bottom, screw the shelves to the supports. It's a simple way to make a unique corner bookcase using a reclaimed door.