Mail Royal

A casket of incriminating letters from Mary, Queen of Scots. The
letters would be an instrument of persuasion - or blackmail as others
might call it. Whoever held them had a hold over the James VI King of
Scots, now also King James I of England.

But for Andrew, seventh Lord Gray, laying hands on the letters was not
going to be easy. They were secure in the half-ruined Fast Castle that
clung to the sea-cliffs of Berwickshire. A castle that no one could
enter if they were not welcome.

And so young David Gray, his bastard nephew, was sent on a perilous
mission...

A thrilling historical novel of 17th century Scotland from Nigel Tranter.