A useful article on finite geometry,
"21 – 6 = 15: A Connection between Two Distinguished Geometries,"
by Albrecht Beutelspacher, American Mathematical Monthly ,
Vol. 93, No. 1, January 1986, pp. 29-41, is available for purchase
at JSTOR.
This article is related to the geometry of the six-set.
For some background, see remarks from 1986 at finitegeometry.org.