The Timothy Brown book is good, although at heart, it's an ethnographic study of the Contras. The author is sympathetic to the Contras; his thesis is(roughly) that they were highland peasants fighting the latest chapter in a long struggle with upper and middle class whites from the Pacific coast, and that they've been unfairly tarred by the integration of ex-Guardia Nacional types(who Brown blames for most of the Contra excesses) and indigenous milpistas into the FDN. He makes a compelling case, and makes use of a lot of primary sources, in particular, previously unreleased FDN/MILPAS archives.