The god of the hive : a novel of suspense featuring Mary Russell and Sherlock Holmes / Laurie R. King.

Enhanced descriptions from Syndetics:

In Laurie R. King's latest Mary Russell--Sherlock Holmes mystery, the acclaimed New York Times bestselling author delivers a thriller of ingenious surprises and unrelenting suspense-as the famous husband and wife sleuths are pursued by a killer immune from the sting of justice.
 
It began as a problem in one of Holmes' beloved beehives, led to a murderous cult, and ended-or so they'd hoped-with a daring escape from a sacrificial altar. Instead, Mary Russell and her husband, Sherlock Holmes, have stirred the wrath and the limitless resources of those they've thwarted. Now they are separated and on the run, wanted by the police, and pursued across the Continent by a ruthless enemy with powerful connections.

Unstoppable together, Russell and Holmes will have to survive this time apart, maintaining tenuous contact only by means of coded messages and cryptic notes. With Holmes' young granddaughter in her safekeeping, Russell will have to call on instincts she didn't know she had. But has the couple already made a fatal mistake by separating, making themselves easier targets for the shadowy government agents sent to silence them?
 
From hidden rooms in London shops and rustic forest cabins to rickety planes over Scotland and boats on the frozen North Sea, Russell and Holmes work their way back to each other while uncovering answers to a mystery that will take both of them to solve. A hermit with a mysterious past and a beautiful young female doctor with a secret, a cruelly scarred flyer and an obsessed man of the cloth, Holmes' brother, Mycroft, and an Intelligence agent who knows too much: Everyone Russell and Holmes meet could either speed their safe reunion or betray them to their enemies-in the most complex, shocking, and deeply personal case of their career.