The Other Daughter by Lauren Willig

After her mother passes away, governess Rachel Woodley discovers the father that she thought died when she was little is still alive. He is actually the Earl of Ardmore, and he has an estate called Carrisford. Feeling angry and deceived, Rachel joins forces with Simon Montfort, a reporter who agrees to introduce her into high society disguised as his cousin in return for an article about Rachel and her family later on. Navigating the world of late night parties proves to be not too hard, but coming face to face with her half-sister and eventually her father is far trickier. The Other Daughter, set in 1920's London, is full of secrets and romance. Perfect for readers who enjoy Daisy Goodwin.