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