Assassin in the greenwood (hugh corbett mysteries, book 7) - a medieval mys

In the summer of 1302 the famous Robin of Locksley, popularly known as Robin Hood, has gone back to his outlaw ways in Sherwood Forest where he battles against royal authority, culminating in the barborous massacre of royal tax collectors and the mysterious murder of Sir Eustace Vechey, one of the sheriffs of Nottingham. Corbett and his two faithful servants Ranulf and Maltote are sent to Nottingham where they find fresh mysteries: why are three arrows shot into the air above Nottingham Castle on the 13th of every month? Who is the traitor in Nottingham Castle? And why have the French despatched an agent to assassinate Corbett?