This is the third book in King's Mary Russell / Sherlock Holmes series, and the more I read, the more I like the series. Russell is intelligent, lively, interesting and feisty - a worthy match for Holmes. King has said that Russell is what Holmes would be if he were female, born in the 20th Century and interested in theology (or words to that effect). In my view, that aspect of Russell has become clearer as the series has progressed. It is also what makes the pairing of the very young Russell and the aging Holmes both understandable and acceptable. Russell, a woman born in 1900, is representative of a new era. Her age and her gender are central to her importance as a character, and are just as important as the fact that her intellect is equal to that of Holmes.
If anything lets this novel down, it's the resolution of the mystery. A resolution which depends on the victim taking elaborate precautions to write and hide a new will because she "felt" that she was in danger felt a little contrived. And it may just be that I finished reading the book when I was tired, but it was not completely clear to me that discovering the secret of the box and the will actually helped nail the villains.. But I quibble. In reality, what makes this book and the series as a whole worth reading is not the mystery, but the characters and the setting, as well as the atmosphere they help create.
For me, the highlight of the novel was the cameo appearance of Peter Wimsey in Chapter 17. There he was, Lord Peter to the life! It made me wonder whether the Sayers estate would have done better to get King rather than Jill Paton Walsh to complete Thrones, Dominations, is waiting in the pile.