A CERTAIN DARKNESS continues Anna Lee Huber's historical mystery series, Verity Kent.  This is book number six in the WWI historical series, featuring the intrepid former British Intelligence agent Verity Kent and her war hero husband Sydney.  This series takes place in Downton Abbey-era post-war England, and Huber continues to do a bang-up job of bringing the period to vibrant life.  Readers would be best suited to enjoy this delightful series in order.   Sydney is called to France to assist the French police in interrogating a captured woman collaborator who refuses to talk to anyone but her prior contact, Sydney.  The collaborator is found dead in her cell from an apparent suicide, but not before she leaves clues with Sydney and Verity regarding papers in her possession that may detail a sunken German ship filled with a fortune in gold.  Whilst there, Verity is contacted by her prior Secret Service boss to locate sensitive documents stolen from a murdered Belgian informant.  The Kents travel all around war-torn Europe to track these two sets of papers, dodging bullets and winkling out many people's secrets.  The descriptions of devastated cities and countryside are heart-wrenching, making me so grateful for my current time and place of relative safety.   The indomitable Verity and Sydney continue to pursue evidence against Lord Ardmore, whom they know to be a traitor to Britain but do not yet have prosecutable evidence of.  The pursuit of Ardmore is the overarching story linking the books, and the shifting sands of loyalties to country and particular countrymen continue to cloud the Kents' quest.  All of the main characters are spies, current or former, or the occasional war hero with intelligence leanings, and they are a very much tricky bunch which makes for fun reading.  Watching Verity parse the truth from the many layers of lies is so gratifying, too!   This is the first book in the series I feel has gotten somewhat bogged down in a lot of WWI minutiae.  Huber has done oodles of research for this series, and it shows in the weft and weave of the story with its many details.  The sheer volume of data provided here, especially near the end of the book, detracts from the overall flow of the story.  Huber has such a deft hand in drawing her characters, though, making them so relatable and captivating, that this carries the story forward despite the background of info dumping.  Huber's A CERTAIN DARKNESS will delight both historical mystery fans and war buffs alike, with a mix of character-driven action and intrigue.