I second Katherine by Anya Seton, that really is such a romantic novel pruners. And her research was very sound so it's not just a souffle of houppelandes and chivalry.
Also love the Dorothy L Sayers novels - Gaudy Night will almost make you expire with passion, although it is best if you read Strong Poison and Have His Carcase first
I Capture the Castle, by Dodie Smith, is sweetly romantic, and so is Elizabeth von Arnim's The Enchanted April