Charles Wells is Professor Emeritus of Mathematics at Case Western Reserve University; his research interest has been in group theory, category theory and its connections with theoretical computer science, in mathematical discourse, and in the problems of teaching and understanding mathematics.
Toposes, Triples, and Theories with Michael Barr (web)
on topos theory, monads and algebraic theories;
Category theory for computing science with Michael Barr (web)
on category theory.