Luck Is No Lady Fallen Ladies Series, Book 1.

He's the dangerously charming owner of a gambling hell. She's the brilliant daughter of a gentleman drowning in debt. Forced to use her talent for mathematics to save her family, Emma finds herself drawn from London's glittering ballrooms deep into its gritty underworld...and discovers there the pleasure of falling from grace. Emma Chadwick always assumed she'd live and die the daughter of a gentleman. But when her father's death reveal...