I've been reading a book by Margaret Visser "The Geometry of Love" in which she delves into the historic, artistic and symbolic and entymological background and significance of every item, including architectural elements, of a small church founded in the 7th century, outside the walls of Rome. The octagon was a very symbolic shape for the people of that time period as it combined the square, representing the physical and earthly ( 4 directions, 4 elements etc) with the circle representing heaven (eternal, unending, perfect.)
The book is very detailed but not dry as you might expect.