If you're looking for textbooks, Abstract and Concrete Categories—The Joy of Cats[0] by Adámek, Herrlich, and Strecker provides a nice introduction and is freel

If you're looking for textbooks, Abstract and Concrete Categories—The Joy of Cats[0] by Adámek, Herrlich, and Strecker provides a nice introduction and is freely available as a PDF download. [0] http://katmat.math.uni-bremen.de/acc/acc.pdf http://katmat.math.uni-bremen.de/acc/acc.pdf