If forced to take only 4 books to a desert island, I would choose:
Sharon Kay Penman’s “Sunne in Splendour” – IMO the best novel about the 15th century Wars of the Roses and Richard III. Sharon’s research is second to none. I’ve read it several times, and learn something new every time.
Anya Seton’s “Katherine”- one of the first historical novels I ever read, with a wonderful portrayal of Edward III’s court and the history of the later 14th century. Again, superbly researched, and I totally fell in love with John of Gaunt!
John Jakes' 'North and South' trilogy - and if you won't let me take the whole trilogy, I'd choose the 2nd volume, 'Love and War' covering the 5 year panorama of the Civil War, which will remind me of my visits to the battlefields of Virginia, Maryland and Pennsylvania.
And okay, the 4th one might just have to be Pride and Prejudice, so I could dream of Colin Firth, the consummate Mr. Darcy!