It's hardly fantasy, but it's certainly medieval, and it's a darn good story to boot-the book to which I refer is, of course, Walter Scott's Ivanhoe. A classic yarn of gallant knights and dastardly would-be usurpers, it's definitely worth reading, though be warned-Scott can veer towards the verbose at times.
LotR is (in my humble opinion) a superb book, a sublime tale in a wonderfully-crafted world whose mythology surpasses any other. On that subject, read Tolkien's The Silmarillion as well, a truly epic tale that makes LotR look like a walk in the park! Such is Tolkien's imagination and mastery of fantasy, he has even shaped a creation myth utterly unlike any other, to give this book the impression of a true chronicle.