If you're looking for the history around eugenics, I'd advocate for spending some time in education history of the era. That is, a number of those who were involved in the eugenics movement saw it as part and parcel of the efficiency and scientific management movements that were all the rage in schools in the early 1900s. So, there's a fair amount of crossover among key figures between the movements.

Likewise, the history of the Indian Boarding Schools, while not explicitly about eugenics, is deeply related. In effect, the schools were about deliberate actions to eliminate certain biological or social traits in existing people as well as their future children. Many of the men who were advocates for the schools also held leadership positions in education systems and would later become advocates for achievement and personality tests created by those from the eugenics movement.

Two books from education history that I'd recommend:

• Someone Has to Fail: The Zero-Sum Game of Public Schooling by David Labaree
• The Big Test: The Secret History of the American Meritocracy by Nicholas Lehmann

Native American Boarding Schools by Mary Stout is sitting in my TBR stack but it came well recommended.

Jonathan Spiro's biography of Madison Grant (a leading American scientific racist of the late 19th-early 20th centuries), Defending the Master Race, is not only a very interesting insight into Grant and his works, but also includes helpful sections which go into general eugenic theories which were prevalent at the time and which influenced Grant, as well as how Grant and those like him influenced later eugenicists and scientific racists.