What is the main conflict in The Thirteenth Tale by Diane Setterfield?

The main character, Margaret Lea, is invited to write the life story of bestselling author, Vida Winter, who has spent her entire career making up her past. Margaret is hesitant, convinced that the master storyteller will lie to her just like she lied to all the others. However, lured by Miss Winter's Victorian style of writing, Margaret takes the contract and quickly realizes that by learning Miss Winter's secret she will be forced to confront a secret of her own.