Daniel Kevles, In the Name of Eugenics, is the classic text. I usually augment it with Diane Paul's Controlling Human Heredity, which is also good.

It's important to keep in mind that in the 1920s, support for eugenics in principle enjoyed very wide support, across the political spectrum. It was a shorthand for "studying human heredity and applying it towards social and individual ends." It did not necessarily entail an endorsement the coercive, Nazi-like policies we have come to associate with the term exclusively; many of the "in principle" supporters did not support some of the nastier manifestations of eugenics in practice. It was a Progressive-era approach that was favored by such a wide grouping of people — conservatives, socialists, scientists, women's rights, civil rights, etc. — that referring to as "obviously dangerous" is sort of a bit of an ahistorical statement. You have to look closely at the actual beliefs, associations, advocacies, etc., to tell who is arguing for coercive forms of eugenics, and who is basically just arguing for what would later be called (in an explicit attempt to move away from the "eugenics" framework and stigma) genetic counseling. Some of the self-proclaimed American eugenicists really were proto-Nazi — the Madison Grants, Harry Laughlins, and Lothrop Stoddards of the world — and some were not, even if one can argue, in retrospect, that all eugenics ideology leads to rather nasty places (it is, at a minimum, ableist by definition, and has tended to end up having very nasty natural affinities with racism, sexism, and classism).