For the historical novelist, reference books are usually a necessity. I was lucky enough to have studied the period I was writing about in graduate school, although I quickly learned that what I knew wasn't necessarily useful to me. I knew a lot about the basic history, major players, and literature, but I needed to know details about fourteenth century food, clothing, armor, weaponry, and so forth. I used my library extensively (thank you, King County Library System!) but I did buy a few books, especially if I had seen them at the library and knew they'd be useful.
For Edward & Isabelle's life, I relied heavily upon The Three Edwards by Thomas Costain, Edward II by Harold Hutchinson, and my unpublished thesis. As a general introduction to daily living in the medieval period I read The Age of Faith by Will Durant. I know, it's old and it has some errors, but for an encyclopedic view of the time you can't beat it. Joseph and Frances Gies's series on medieval life was also extremely useful, especially Life in a Medieval Castle. The Writer's Guide to Everyday Life in Medieval Europe by Sherrilyn Kenyon was a great find; they have an entire series on various time periods. I read a lot of books on fourteenth century armor and weapons. I also bought The Knight and Chivalry by Richard Barber and Armies and Warfare in the Middle Ages by Michael Prestwich. I also took out a lot of books on castles and royal houses in England. A friend came back from England with The Tower of London: A 2000-year history by Lapper and Parnell. Prince Edward of Wessex has a beautifully illustrated book that helped a great deal. I spent so much time in the 941 section of my local library that I still find myself drawn to it even today. The complete list would take a long time to compile and would probably make for pretty dull reading, but you get the gist. It's a good thing I like libraries.