I always used to use the (alwayssssssssssssssssssss empty) reading/extra classroom on the sixth floor of Ross Hall (the Department of History). Big desk to keep you hidden behind, soft chairs, you can lean them over and use the back of it as a reverse pillow, cool in the summer, warm in the winter, and rather dark/dim.
Other than that, I would find an empty meeting room in the Memorial Union (the ones on the third floor, back towards the parking deck, are good, so is the one at the end of the NE hallway looking out towards the Campanile). The library is *okay,* but there was always just enough traffic to make it suboptimal, for me. The stacks are really uncomfortable with those hard, tiled floors, too, you need yourself some carpet and soft chairs to do this right.