Horst Herrlich, George Strecker, Categorical topology – Its origins as exemplified by the unfolding of the theory of topological reflections and coreflections before 1971 (pdf), pages 255-341 in: C. E. Aull, R Lowen (eds.), Handbook of the History of General Topology. Vol. 1 , Kluwer 1997 (doi:10.1007/978-94-017-0468-7)