Deciding whether there are too many books on this particular Queen must depend of course on what the latest author has to say, given the very many others in or out of print.

From 1568 until her execution at Fotheringhay Castle in 1587, Mary, Queen of Scots was a prisoner. As Dr Scott writes, travelling through Mary’s years in captivity is “a well-worn path”.

Yet, from her chambers, Mary wrote countless letters, and many of these letters were encrypted using complex ciphers, as her surveillance was so close, and known to be so. That added complexity was necessary for writers to prevent communications from being understood easily by those who did not have her best interests at heart. The enduring importance of the letters, and presumably others destroyed or now simply lost, was clear; even in captivity, Mary could exert her influence, and she was seen as a threat.

More than 400 years after Mary’s death, the discovery in Paris of further encoded letters (initially believed to be anonymous Italian writings) has advanced this area. For their own interest, three amateurs worked to break the code and succeeded. That has renewed interest in the encrypted correspondence of Mary while in captivity.

This new book has helpful lists of the dramatis personae and also a chronology of the life of Mary, although the former list is at the front of the book and the latter at the end. General readers may include students of the history of espionage, or diplomacy, and there is much to reflect on, even though the events were nearly 450 years ago. The ingenuity in devising codes and the difficulty in defeating those watching Mary are explained, as are the complexities of moving highly sensitive papers between correspondents.

The style of the correspondence varied: many letters were written by a secretary, and merely signed by Mary, others were signed by her with post scriptum notes added by the signatory.

The history of the captivity of Mary is thus retold with more than the usual detail of how planning was conducted by the secret means with cryptic letters. Moreover, there was then a style of letter-writing that was not merely communicating but was embedded with social and interpersonal cues, and deference to status.

Identifying new letters and breaking the code has added a new aspect to the imprisonment of Mary. This aspect of the story of the captivity is well told and supplemented with an excellent selection of colour photographs.

Captive Queen: The Decrypted History of May Queen of Scots. Published by Michael O’Mara Books, 304 pp, £20.