Wow, those are tough options. I would say that the closet is the worst choice, and the top of the bookshelves is the best, provided that (1) this does not raises the speakers too close to the ceiling, and (2) you can arrange for the speakers to point down at you.
One way to get the speakers pointing down is to wall mount them with professional style, articulating monitor stands. I have been in some small mixing suites with that arrangement and the sound was better than you would think. Good luck.
One way to get the speakers pointing down is to wall mount them with professional style, articulating monitor stands. I have been in some small mixing suites with that arrangement and the sound was better than you would think. Good luck.