Crop Circles: History, Research And Theories by Eva-Marie Brekkesto
Sacred Geometry: Symbolism and Purpose in Religious Structures by Nigel Pennick
Recommended Read 'Sacred Geometry' by Nigel Campbell Pennick, currently published by Abe Books, is a classic starter to scared geometry. Indeed it was the first book I bought on sacred geometry! It is only a short book with some basic sacred geometry and light on examples. A nice quick read.
Nigel is a marine biologist, who writes about a variety of topics - over 50 books and papers on occultism, magic, natural magic, divination, subterranea, rural folk customs, traditional performance, Celtic art, runosophy, European arts and crafts, buildings, landscape, customs, games and spiritual traditions. But Nigel is most known for his knowledge of sacred geometry, geomancy, labyrinths, spiritual arts and crafts, esoteric alphabets and Germanic runic studies. He is also an artist, notably of Celtic artwork and creating labyrinths.