Because it was not mentioned until now, one of the following might probably come in handy: - Basic Category Theory for Computer Scientists by Benjamin C. Pierc

Because it was not mentioned until now, one of the following might probably come in handy: - Basic Category Theory for Computer Scientists by Benjamin C. Pierce (from "Types and Programming Languages" fame) - Practical Foundations of Mathematics, by Paul Taylor (has other interesting work on his homepage as well; http://www.paultaylor.eu/~pt/prafm/ http://www.paultaylor.eu/~pt/prafm/; somebody on HN pointed me to it, thanks!--Chapters 4,5,7)