A topic of interest to me also - a convinced Friend, genealogy research has found Quaker ancestors. Also, it's revealed non-Quaker slave-holder ancestors. DNA tests have also introduced me to several relatives who may have been descendants of those slave-holding ancestors.
One book I'm reading now, American Colonies by Alan Taylor, talks a bit about Quakers in general, and touches on slavery and Quakers. He says William Penn had a few slaves - this back in the late 1600's in Pennsylvania.
He also says Philadelphia Yearly Meeting in 1758 barred Quaker slaveholders from leadership in Yearly Meeting - which means there were Quaker slavekeepers there and then. In 1776 it barred them from membership. So, nearly 100 years to bar them from leadership - another 20 to bar them from membership. Not an easy decision, evidently.
This is very Quakerly :) 1) Quakers being among the first to do the "right thing" in social issues 2) it taking Friends a long time to come to agreement about what the "right thing" is.
I'd heard for some time that it took Friends 100 years to agree that slavery was wrong. Would recommend this book. Penguin is the publisher. Not a lot about Quakers but it puts it all into the times and reasoning going on.