The Red Queen by Philippa Gregory (The Plantagenet & Tudor Novels #3)

Child-bride of Edmund Tudor, although widowed in her early teens, is determined to infiltrate the house of York under the guise of loyal friend and servant. It's the only way to ultimately ensure that her only son, Henry Tudor, triumphs as King of England.
Through collaboration with the dowager, Queen Elizabeth Woodville, Margaret agrees a betrothal between Henry and Elizabeth's daughter, thereby uniting the families and resolving the Cousins War once and for all by founding of the Tudor dynasty.