Selina Cadell has played 3 characters on Midsomer Murders (1997) Venetia Butts, Eleanor Crouch and Phyllis Cadell.
The idea of a book that holds clues to a real treasure is probably a reference to the bestselling 1979 book "Masquerade" by Kit Williams.
The name of the location Mods Mire where this is set is an anagram of Midsomer.