1. Andreas Blass, The interaction between category theory and set theory, pp. 5–29 in Mathematical applications of category theory, 1983. MR 85m :03045.
2. Samuel Eilenberg and Saunders Mac Lane, General theory of natural equivalences, Trans. AMS 58 (1945) 231–294. MR 7 109d.
3. Peter Freyd, Abelian Categories, Harper and Row, 1964. Out of print, but accessible online, with a new Foreword, at
http://www.tac.mta.ca/tac/reprints/articles/3/tr3.pdf
. MR 29 #3517.
4. F. William Lawvere, The category of categories as a foundation for mathematics, pp. 1–20 in Proc. Conf. Categorical Algebra (La Jolla, Calif., 1965) ed. S. Eilenberg et al. Springer-Verlag, 1966. MR 34 #7332. (Note corrections to this paper in the MR review.)
5. Saunders Mac Lane, Categories for the Working Mathematician, Springer GTM, v.5, 1971. MR 50 #7275.