Get any decent Byzantine history from Amazon - I recommend Vasiliev or Ostrogorsky. Gibbon was a splendid writer and managed to impress and hypnotise readers with his vision. But at his time people in England knew squat about late antiquity or Christian East. And he had a Deistic pro-Muslim agenda. Read solid history on the subject and you will have many proofs.
Gibbon is a great writer to be valued for his ideas, not because his work was true in factual sense but was true as a didactic phantasy.