The book is called The Shorter Family: England, america, and Africa in the History of a Family by Aylward Shorter. He co-wrote the book with an African American writer named Maggie Taylor. The book was published by Heritage Books. I checked at Amazon and it is out of print but you could call Heritage yourself- 1-800-876-6103 and see if they have a single copy somewhere. Have you contacted the Afro-American Historical and Genealogical Society of Prince George's County? (pgcm.aahgs.org) They may be able to help you get around your wall.
I was just reading an article about "The Pearl" and the article mentioned that " the 1850 Compromise included a law which banned the sale of slaves in D.C. This did not stop private sales but it did mean the free blacks could now live without the fear of being legally captured and sold south." This was a draw for many of the free people in the neighboring states. I have seen the Shorter's disappear from my MD research and appear in D.C. Now I understand why.