Comments or suggestions:
Category theory
Jaap van Oosten, Basic category theory. here.
Andrea Schalk and H. Simmons, An introduction to category theory in four easy movements. here
Daniele Turim, Category theory lecture notes .Based on Mac Lane’s book (1998). here,
Jiri Adamek?, Horst Herrlich?, and George Strecker?, Abstract and concrete categories: the joy of cats.
Michael Barr? and Charles Wells?, Toposes, triples and theories. here
A. Martini, H. Ehrig, and D. Nunes, Elements of basic category theory.here,
Max Kelly, Basic concepts of enriched category theory (tac reprints)
Saunders Mac Lane, Categories for the working mathematician, 2nd ed.
Steve Awodey, Categories for everybody.
Francis Borceux?, Handbook of categorical algebra, vol 1–3.
Kashiwara and Schapira, Categories and Sheaves, Springer (2000)