** spoiler alert **
I enjoyed reading the Beekeeper's Apprentice, and I enjoyed this book too, but I felt that I was cheated in the end.. The fact that Mary Childe's husband was the crook...it seemed like it was randomly pulled out a hat. Perhaps the only thing that saved it for me was that her husband didn't kill all these women because of religion or the feminist movement, it was just for the money. But it was a little bit of a let down.
As for Holmes and his romantic relationship with Russell, I was SPOILED. My friend burst out that Sherlock Homes and Mary Russell end up marrying each other and I thought she was joking because the first thoughts that jumped into my head were a) Isn't he fifty something???, and b) I imagine him as a father figure to Russell, since she lost her father (along with her entire family) years before. So I was surprised that King would actually set the two up even if Russell was essentially meant to be the female version of Holmes.
I ended up not minding it so much after I finished the book. Russell and Holmes noted the thirty seven age gap, but what makes their relationship work and why they are attracted to each other is, as Russell explains in the book: "For me, for always, the paramount organ of passion was the mind. Unnatural, unbalanced, perhaps, but it was true: Without intellect, there could be no love." So I guess I didn't mind it too much. :-)