[1] P. FREYD: Abelian categories. New York, 1964. Zbl 0121.02103

[2] P. FREYD: On the concretness of certain categories. - preprint. MR 0271184

[3] V. KOUBEK J. REITERMAN: Factorcategories of the category of sets: description and concretness. to appear.

[4] V. KOUBEK: Set functors. Comment. Math. Univ. Carolinae 12 (1971), 175-195. MR 0286860 | Zbl 0217.06803

[5] V. KOUBEK: Set functors II - contravariant case. Comment. Math. Univ. Carolinae 14 (1973), 47-59. MR 0318257

[6] S. MAC-LANE: Kategorien. Berlin - Heidelberg - New York, 1972. Zbl 0243.18001

[7] V. TRNKOVÁ: On descriptive classification of set functors, I, II. Comment. Math. Univ. Carolinae 12 (1971), 143-175 (Part I); 346-357 (Part II). MR 0294445