These days one often comes across people in other (ie. usually not physics) departments who have some interest in category theory. I already mentioned Lawvere and Rosebrugh's book, Sets for Mathematics, to the lovely young philosopher who is studying Frege, amongst other things. And I suspect that Ross Street's book on Quantum Groups is also useful. Poor Carl is presently struggling with the ubiquitous text by the late Mac Lane. This book is very good, but perhaps not for the beginner. My favourite is Sheaves in Geometry and Logic, but that betrays a bias towards topos theory.
Aside: Now you can support Abtruse Goose with this groovy cap. I thought of merchandising for funds, but unfortunately it is entirely against my green anti-materialist ethos. Maybe I'll buy a cap though. AG deserves it.