Michael Barr and Charles Wells has put up a newly revised version their formerly Springer Verlag published book "Toposes, Triples and Theories" online for free download; see http://www.cwru.edu/artsci/math/wells/pub/ttt.html or the newsgroup sci.math.research. They differ in terminology, instead of MacLane's "monad", they use "triple", which may be unaccustomed to Haskell users. But apart from that, it might be a good reference. Hans Aberg
