Categories Resources

Books:

Conceptual Mathematics: A First Introduction to Categories by F. William Lawvere and Stephen Hoel Schanuel

Category Theory for Scientists by David I. Spivak

Abstract and Concrete Categories: The Joy of Cats by Jiří Adámek, Horst Herrlich, and George E. Strecker

Websites:

Make Category Theory Intuitive webpage

Wikipedia's Portal for Category Theory

The nLab