Answer:
In 1871 Still became the first anti-slavery activist to document the experiences of fugitive slaves in his book The Underground Railroad, a work which explained the story often in the words of the participants in the effort to escape slavery. The book provided intimate detail on the workings of conductors like himself but it also provided numerous letters and testimonials from fugitive slaves to Still either requesting assistance or thanking him for his efforts. Even today, The Underground Railroad remains a major source for understanding this active and concealed resistance to slavery.
William Still continued to campaign for civil rights in Philadelphia as a researcher, writer, and activist until his death in 1902.