You can search this book online on Google that has some items about John W. Headley, Mary Overall & her sister Sophia etc..
Spies of the Confederacy--By John Bakeless
http://books.google.com/books?id=h77qZ1FR2DwC&pg=PA198&dq=mary+overall&sig=aUYC7hOQxe9dS0R_5zFRTMHQiAY
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Gay