Seconding Guy Gavriel Kay, which is supposedly really very good, but I haven't managed to read yet.

Harry Turtledove wrote a whole series of alternate histories based around the Byzantine Empire persisting - the short stories are collected in "Agent of Byzantium" - but I don't know that I'd actually recommend them. (In fact his AHs were just brought up in a comment thread as being pretty faily, which accords with my memory.)

Other than that, I think I've mostly got stupid Templar conspiracies, and books where people travel the spice trade and stop off in Byzantium along the way.