Are there any good sources or books on them? Where were they usually located? If they did exist?
I know of at least one case. Zephaniah Kingsley ran the Kingsley plantation on Fort George Island north of Jacksonville, Fl. He married his slave, Anna Kingsley and had 9 children with her. Florida was owned by Spain at this time. After Florida changed hands to the US, he began campaigning for rights of free blacks and other humanitarian efforts. Facing discrimination and criminalization of interracial marriage, they fled to Haiti and lived peacefully.
Zephaniah Kingsley seemed to be an exceptionally lenient and kind slaveowner. After his death, there was a legal battle between Anna and his family for his property. Amazingly, she won and retained ownership of the plantation.
Most of this is from memory from doing the Kingsley plantation tour a few times, but I'll add a source: