The Enigma of a Widow - Linda Rae Sande

Having lost her husband in the Battle of Ligny, Lady Lydia Barrymore is determined to resume her work for the Foreign Office when her mourning period is over. She's spent a year solving puzzles and assembling dissected maps to maintain her skills. Her first assignment has her perplexed, though—do what she must to help a fellow operative recover his sanity. Although she finds the man rather beautiful, Sir Donald has also proved most annoying.

Newly knighted Adonis Truscott returned from the Continent with a tendency to get lost in his thoughts. His frequent episodes of staring into space have his sister claiming he's a candidate for Bedlam—and he's not about to argue. He doesn't always remember where or when he was when he recovers, but he remembers he made a promise, and he's determined to keep it. 

A promise to provide protection for Lydia, whether she wants it or not.

When a puzzle’s directions require Lydia to solve it with the help of Adonis, she discovers the man harbors secrets that may be impossible to reveal. With her own sanity in jeopardy—a year-long mystery involving her late husband may be more annoying and dangerous than an errant knight—Lydia will have to piece together a solution that suits them both in The Enigma of a Widow.