I bought The Beekeeper's Apprentice a while back as part of a B2B1F sale and just never got around to reading it. Well, @mary_russell started tweeting bits of plot from her books since there's a new one coming this fall and they looked interesting...so I started reading last night.
And finished at 3am. Very fun to read, very hard to put down. A great character study aside from a good mystery (with a hint of one in the middle that Mary skirted around that I think is actually book 5, O Jerusalem). Very evocative of the Holmes voice via Conan Doyle without trying too hard and overdoing it. I loved Mary - sharp as a tack, stubborn, and more than Holmes's match. Much better than boring old Irene Adler. Having read a few spoilers, I do have to say that the age gap between Holmes and Mary is only very weird when it gets emphasized.
Also a good read for the Downton Abbey fans since the WWI setting from the point of rations, working in the fields, etc. is very well-described from the vantage of a very intelligent, Oxford-bound teen.