Although not specifically about your topic, this is an important book for understanding the Nazi justification and its role model:
Hitler's American Model: The United States and the Making of Nazi Race Law by James Q. Whitman. In the same vein,
Three Generations, No Imbeciles: Eugenics, the Supreme Court, and Buck v. Bell by Paul A. Lombardo. Again, this is American focused but the decision provided a much desired rationale for what ultimately became the Nazi holocaust.