The novel I chose for my ISU is Philip Gregory's The Other Boleyn Girl. It is set in the 18th century, when King Henry VIII is in power over Great Britain, and is about his second wife, Anne Boleyn.
While Gregory was not born in the 18th century, (she was born in the 1954), she has studied history at the University of Edinburgh. She later received her doctorate in 18th-century literature, and has even taught history at several universities. Gregory also claims that she has a commitment to historical accuracy, so that each of her novels are accurate, and not full of lies and made up facts.
While she hasn't directly had any experience as a noblewoman trying to get within the King's private circle, but having studied history, more importantly 18th-century history, and has even taught it, and always makes sure to be accurate, I would say Gregory is a credible author.