THE GOD OF THE HIVE, by Laurie R. King
They return to London, not knowing if the other is alive, and find that a new "god of the hive" has arranged Mycroft Holmes' disappearance. Skulking from bolt-hole to bolt-hole, Mary lock-picks her way into her brother-in-law's apartments, as well as a couple of places she should have known better than to attempt. Sherlock, master of disguises, keeps missing his wife, literally and figuratively. The denouement is satisfyingly chilling. This series could of course be read as stand-alones, but why deny yourself the pleasure of reading the entire dozen books. Start with The Beekeeper's Apprentice and you'll be hooked. It's a 4.5.