I've been looking for this for a while and have had no luck online. Have you ever seen it? I would like the whole book if it can be located. Its mentioned in Allan Austin's African Muslims Antebellum America: A Sourcebook. At least a dozen authors mention it later without ever seeing it. I tried to email Allan Austin, but he hasn't responded to my emails or those from other scholars in over a year. I also contacted the American Historical Society and they suggested I post here.
Louis Harlan, "The Prince: The Biography of a Slave" (mostly a condensation of Sydnor) , and B. Marie Perinbam, "Abd al-Rahman: A Critical Note," in Job Ben Solomon and Abd al-Rahman: The Stories of Two Men in Slavery (Washington, D.C. American Historical Society, 1970).