A beautiful blue death / Charles Finch.

Enhanced descriptions from Syndetics:

On any given day in London, all Charles Lenox, Victorian gentleman and armchair explorer, wants to do is relax in his private study with a cup of tea, a roaring fire and a good book. But when his lifelong friend Lady Jane asks for his help, Lenox cannot resist another chance to unravel a mystery, even if it means trudging through the snow to her townhouse next door.
One of Jane's former servants, Prudence Smith, is dead -- an apparent suicide. But Lenox suspects something far more sinister: murder, by a rare and deadly poison. The house where the girl worked is full of suspects, and though Prudence dabbled with the hearts of more than a few men, Lenox is baffled by an elusive lack of motive in the girl's death.   When another body turns up during the London season's most fashionable ball, Lenox must untangle a web of loyalties and animosities. Was it jealousy that killed Prudence? Or was it something else entirely, something that Lenox alone can uncover before the killer strikes again -- disturbingly close to home?