The Empty Chair - Baker Street Legacy, #2

Happiness becomes fear and peace becomes peril as an insidious enemy sets off a dangerous game of cat and mouse. Kipling Branson's tumble into love with Grayson Holmes was absolute, but if a long-distance relationship wasn't hard enough she loves a man whose career leaves his life in constant danger. When Grayson's sisters propose a surprise visit to England, Kipling ignores her hesitation and takes the chance.
In the months since the death of Grayson's cousin, a man closer to him than a brother, Grayson has found himself wishing for a more fulfilling and less solitary life. With Kipling, he has a chance. What should have been a peaceful holiday learning to share space turns into yet another life-or-death series of riddles... with Grayson's life the final prize. The Howell family's hatred for the Holmes bloodline has lasted generations.
and now plays out in a game of revenge. Howell has twisted truths, taken lives, and machinated outcomes with the ultimate goal of retribution. His games in Boston failed, but he has a new game to play.