Everybody knows that there are lots of free ebooks available for download. I just stumbled across a site that has some great books relevant to students of Black History and I downloaded a great Thurgood Marshall bio. There are several I already have that you might like such as The Life and Times of Frederick Douglass and the Lerone Bennett Jr classic Before the Mayflower. (There are other categories as well.)
African-American History - Ebooks for Students
https://ebooksforstudents.org/category/african-american-history/