Originally Posted by ATDrake
Not exactly a genre-crosser, but an SF novel which very strongly incorporates medieval history and is quite excellent and highly recommended: Connie Willis' Doomsday Book. A time-traveling Oxford historian researches the Middle Ages directly. This won either a Hugo or a Nebula Award; possibly both.
You can also toss in (for a bit later in England) To Say Nothing of the Dog
, and (soon) All Clear
(no Wikipedia entry yet.)