I haven't had a chance to post on it since last weekend, but I finally got the first phase of the study ceiling painting completed. After putting up the ribs and the coffered sections over each end, last weekend I figured out how to do the skylight in the center. I rolled on a very light flat blue, then painted faux parapets with recessed panels. The panels line up with the coffers and their sides (and the angles for the parapet corners) were set using a string from the center of the skylight so that from the exact center of the room, the perspective is correct. 
That conflicts with the highlights and shadows through the rest of the room, which are correct more or less for one corner of the room, but I thought the skylight really needed to be set for viewing in the center.
I also worked on the lighting in the room quite a bit. Changes included:
- Raising the rope lighting behind the cornice so it casts a brighter light on the ceiling
- Cutting metal half circles to place on top of the the wall sconces so that they don't throw light up on the ceiling - just the bookcases.
- Buying two torchere lamps to throw light on the skylight area to reinforce the illusion that it is daylight. As they throw a lot of glare at eye level, I've smeared the inside of the alabaster globes with mahogany wood filler (a very thick stain-like material used to fill the pores of mahogany when a smooth surface is desired). They need more, and I will probably add some oak to end with with an amber-looking globe.
I'm very happy with the way the skylight turned out, but it really does need clouds. And I have already started on the plan to add a faux parapet above the bookshelves to help fill in the space on the sides
