Maurice Isserman, a Hamilton College historian who specializes in studies of the left, knows that one can't really understand Harrington unless one understands some of the maddening political nuances of the recent past... "The Other American" is mainly about "The Life of Michael Harrington," its subtitle. But given the nature of Harrington's idiosyncratic career, it is also a veritable Zagat's Guide through the left sectarian factions of the last three-quarters of the 20th century.