Goldblatt, R (1984) Topoi: the Categorial Analysis of Logic A clear introduction to categories, with particular emphasis on the recent applications to logic.
Hillman, Chris Categorical primer, Formal introduction to Category Theory.
Awodey, Steven (2006). Category Theory (Oxford Logic Guides 49). Oxford University Press.
Borceux, Francis (1994). Handbook of categorical algebra (Encyclopedia of Mathematics and its Applications 50-52). Cambridge Univ. Press.
Freyd, Peter J. & Scedrov, Andre, (1990). Categories, allegories (North Holland Mathematical Library 39). North Holland.
Hatcher, William S. (1982). The Logical Foundations of Mathematics, 2nd ed. Pergamon. Chpt. 8 is an idiosyncratic introduction to category theory, presented as a first order theory.
Lawvere, William, & Rosebrugh, Robert (2003). Sets for mathematics. Cambridge University Press.
Lawvere, William, & Schanuel, Steve (1997). Conceptual mathematics: a first introduction to categories. Cambridge University Press.
Leinster, Tom, Basic Category Theory, Cambridge University Press, 2014.
McLarty, Colin (1991). Elementary Categories, Elementary Toposes. Oxford University Press.
Mac Lane, Saunders (1998). Categories for the Working Mathematician'. 2nd ed. (Graduate Texts in Mathematics 5). Springer-Verlag.
——— and Garrett Birkhoff (1967). Algebra. 1999 reprint of the 2nd ed., Chelsea. ISBN 0-8218-1646-2. An introduction to the subject making judicious use of category theoretic concepts, especially commutative diagrams.
May, Peter (1999). A Concise Course in Algebraic Topology. University of Chicago Press, ISBN 0-226-51183-9.
Pedicchio, Maria Cristina & Tholen, Walter (2004). Categorical foundations (Encyclopedia of Mathematics and its Applications 97). Cambridge University Press.
Pierce, Benjamin (1991). Basic Category Theory for Computer Scientists. MIT Press.
Taylor, Paul (1999). Practical Foundations of Mathematics. Cambridge University Press. An introduction to the connection between category theory and constructive mathematics.