Beekeeper's Apprentice: or on the Segregation of the Queen by Laurie R. King

Featuring Sherlock Holmes and Mary Russell

In the early years of WW I, 15-year-old American Mary Russell encounters Holmes, retired in Sussex Downs where Conan Doyle left him raising bees. Mary, an orphan rebelling against her guardian aunt's strictures, impresses the sleuth with her intelligence and acumen. Holmes initiates her into the mysteries of detection, allowing her to participate in a few cases when she comes home from her studies at Oxford. The collaboration is ignited by the kidnapping in Wales of Jessica Simpson, daughter of an American senator. The sleuthing duo find signs of the hand of a master criminal, and after Russell rescues the child, attempts are made on their lives (and on Watson's), with evidence piling up that the master criminal is out to get Holmes and all he holds dear. 

Now I know that I am biased as I'm a huge Sherlock Holmes fan and have tried out various series out there that feature Sherlock Holmes but still find this one of my favorite as he seems much more likable.  King does an excellent job of tying in the original stories and then adapting her characters to fill in the gaps.  I've read this one several times and it always seems as fresh as the first time I read it years ago.