There's just something really attractive about the idea of integrating a desk into a bookshelf. Sure, you'll lose some shelving space, but the efficient storage and discreet work area in combination is a small space dweller's dream. Being surrounded by books at arm's length is also a plus. While many of us do not have the luxury of a separate study, converting a bookshelf into a desk+work area could be the next best thing...
She first bought some IKEA Vika Kaj legs that would help prop her desk up. Since the desk would support more weight than a simple bookcase shelf, she needed to add some L brackets to the board that would serve as the desk's main surface. The table top was propped up on the middle shelf of the Billy. Instead of the default height, she chose the height that would fit her the best. In her case, 2 ½ feet worked well. She also decided not to install the Billy's backboard because she'd need to run the cables out of the back of the bookshelf.
She then marked the tabletop at the right spots to mount the table legs. Screw them in and shimmy them to the right height. Since most tables have got some possible adjustments, you should make sure that the height is perfect for the main user of the desk. Once the tabletop with the legs is resting flush on the middle shelf, it's time to screw it in.
This setup is perfect for some ambient lighting. For starters, a few IKEA LED Dioder strips could work well, depending on where they are situated but we'd be inclined to actually install some bright spots under the the shelf that is right above the desk. The LED IKEA Inreda might work well but any number of spotlights might do the trick. The Dioder lights would probably best serve if they are mounted under the desk to serve as ambient lighting while 2 to 3 spots could be mounted on the shelf.