"Many years ago, in my foolish youth, I thought I should never marry. I was quite convinced that strong emotion interfered with rational thought, like grit in a sensitive instrument ... However, I begin to suspect that -- I shall say this quietly -- that I was wrong, that there may be times when the heart sees something that the mind does not. Perhaps what we call the heart is simply a more efficient means of evaluating data." (p.144)Mary Russell, recently married to Holmes and somewhat disturbed by the ennui she perceives in his behaviour, is visited by an old friend from Palestine, who gives her an ornately-carved box containing a letter written on papyrus. Mary -- fully engaged with her own theological research -- is fascinated by the manuscript, which seems to have been written by Mary Magdalene in AD70. Theology irritates Holmes ('he judged it a tragic waste of my mental abilities') but all too soon there's a crime to investigate, and the Holmes-Russell partnership turns its focus, and the complementary skills of detective and academic, on a confusion of misogyny, mental illness, hidden memories and murder.
King drops hints of hitherto-unknown Holmes cases (services rendered to the Royal Order of Nigerian Blacksmiths; snowed in near the Khyber Pass) and cameo appearances of the famous and / or fictional (Mary encounters 'an odd man named Tolkien' in a pub, and I'm sure that's Peter Wimsey playing the clavier). There are also some observations on advances in forensic science since Holmes' heyday: "Things he did that looked crazy thirty, forty years ago are now standard procedure with us," says a police inspector. (p.96)
A Letter of Mary shows Russell in her academic milieu, as well as in a variety of roles and disguises: it also shows the gradual accommodation between a young woman and her much older husband. Holmes is learning to be half of an equal partnership: after running roughshod over Russell's theories, he lauds Watson's 'doglike devotion' but notes that (p.265) "I did not consider this a strength when it came to a permanent partnership". ("Woof," says Russell.)
Mary Russell is very much the focus of the novel, but there's a pleasing sense of cameraderie between her and Holmes, and the investigation is very much a joint effort.