http://www.math.mcgill.ca/triples/
Category Theory at McGill A personal account by Marta Bunge.
Category Theory - an expository article by Jean-Pierre Marquis from the Stanford Encyclopedia of Philosophy.
Categorical Logic, an entry by Joachim Lambek from the Encyclopaedia of Mathematics (Springer).
Category, another article from the Encyclopaedia of Mathematics (Springer).
Categories, Quantization, and Much More - an introduction by John Baez. (You might also explore his series "This Week's Finds in Mathematical Physics" which often has references to logic, category theory, and particularly n-category theory and more n-category theory. You can also look at these slides: Why and What are n-categories?)
Toposes, Triples and Theories A classic text by M Barr and C Wells, now freely available on-line (TAC Reprints). (Also available at the authors' own website.)
Category Theory for Computing Science (Another classic text by M Barr and C Wells, published by the CRM.) Now freely available here, on TAC, and on the authors' webpage.
