A Marriage Most Convenient by Robyn DeHart

There are four ladies in London who have a most unladylike hobby: solving mysteries. No conundrum is too great for these young women. Until they stumble upon the Jack of Hearts, a scandalous rogue who steals jewels from the rich and may well steal the heart of one of their own.
When Charlotte finds herself penniless and compromised by London’s most notorious masked scoundrel she resigns herself to fleeing to a nunnery. Or perhaps just the countryside. Then Edmond Mabson, her best friend’s older brother, swoops in and does the gallant and gentlemanly thing and marries her to salvage her reputation. As they grow closer, Edmond has a secret that could destroy them both.