Camelot Caper — Elizabeth Peters

Jessica Tregarth went to England to visit her grandfather: an invitation which surprised and pleased her. The only link she had with her dead father's family was an antique ring he had brought with him to America. This would be a chance to learn more about who she is; it would he fun.

She's barely off the boat before the chase begins and Jess finds herself playing a deadly game of cat-and-mouse through Cornwall, helped by David Randall, the ingenious author of a series of paperback gothic novels. But even Randall's cleverness may not be enough--the couple doesn't know what the pursuers want...and it is not the obvious.

The original title of this book was HER COUSIN JOHN, released in 1969 by Dell.