LADY ALMINA AND THE REAL DOWNTON ABBEY, and              LADY CATHERINE, THE EARL, AND THE REAL DOWNTON ABBEY, both by Lady Fiona Carnarvon (read about them here)

THE DAUGHTER OF HIGHLAND HALL, and                                             THE GOVERNESS OF HIGHLAND HALL, both by Carrie Turansky

THE UNOFFICIAL DOWNTON ABBEY COOKBOOK, by Emily Ansara Baines (excellent review here)