I read this book, more or less by accident, not particularly famous and was dazzled. Wow. Can't think of what to say, but will try.
The novel is set in an alternate history Europe, where either Constantine never converted to Christianity or Julian established the equality of all faiths, and the Byzantine Empire never declined, but in fact by the middle of the XVth century controls most of Eastern Europe and is trying to get as much of the West as possible. And magic works, and vampires exist also. I don't usually like alternate history, the real historical characters usually look unlikely next to the alternate bits, but this novel handled it perfectly, and the real historical characters of the XVth century (Richard III of England, his mother, and brothers, the Earl Rivers, Louis XI of France, the Medicci, the Duke of Urbino) are a joy to read about if you have met them before.
Great novel, deserving of a far better review than this one.