Ta Da! One tall bookcase...
It's the same series as my desk, so the colour matches perfectly! It only took me about an hour to build it, counting a little break to let my neighbors downstairs know I would be hammering nails up here. They appreciated the courtesy.
I have several more books and things to fill the shelves with, but I staged this photo like this just to give a general idea of how the new bookcase looks in the den. I can't leave that many open spaces for the cat to climb into.