I recently stumbled upon a curious old volume on Google Books called Housing the Sunday School, which you may wish to read if you are on the planning committee to design a Sunday School building, but it reminded me of a really excellent book I read a long time ago by 19th Century art and architecture critic, John Ruskin, called The Seven Lamps of Architecture.
I'm sure there are excellent newer books with a wider perspective, but I thought Ruskin's book was excellent. Even though it isn't exclusively about church buildings, a lot of it is. I highly recommend it, especially if you are on a church building committee.