Katherine by Anya Seton -- a very readable novel about Katherine Swynford, the mistress of the third son of English king Edward III, whose descendants later ruled England. It has a strong love story, lots of drama, a bipolar mother-in-law, and the Black Plague.
Any of the Plantagenet books written by Thomas Costain, including The Three Edwards, The Conquering Family and the Last Plantagenet. Also his novel, Below The Salt.