M.F.
Hey J.,
I don't have any reading material for you, but my first thought is that in the middle, they are inherently safer from rear, side and frontal collisions. They're perfectly "cusioned" in the middle for it. I have 3 kids, all on car seat or booster, and they all sit next to eash other in a little row. I'd say an advantage to having them in the middle, too, is that during long car rides, you are able to tend to them easier than if they are in the third row. Just food for thought. :O)
Luck!
M.