The following 2 books have been recommended as being very helpful for researchers of the US Virgin Islands:

"Virgin Island Exodus: the United States Citizen's Identification Card System of 1918 and the Post-Transfer Quest for Opportunity Under the United States Flag", written and compiled by David W. Knight and Laurette de T. Prime, 1998, ISBN 1 891610 02 3. (On the back cover: Soon after the transfer of the Virgin Islands from Denmark to the United States, the issue of the status of the islands' residents was raised.... The photo identity card travel pass...allowed Virgin Islanders unrestricted access to Puerto Rico and the United States mainland as American nationals...")

and

"St. Thomas 1803 Crossroads of the Diaspora: The 1803 Proceedings and Register of the Free Colored inhabitants in the town of Charlotte Amalie, on the island of St. Thomas in the Danish West Indies", edited and compiled by David W. Knight & Laurette de T. Prime, 1999, ISBN 1 891610 03 1.

Both books are listed as available from:

Little Nordside Press
PO Box 9767
St. Thomas, US Virgin Islands 00801