Is there a more modern, definitive book on the fall of Rome. Or, even better, on the entire history of the Roman Empire?
Until someone replies with a proper reading list, you might want to read these two answers by u/shlin28 and u/LegalAction which deal with Gibbon's shortcomings and suggest alternative reading.
The most likely recommendations you're going to get are Heather's The Restoration of Rome, O'Donnell's The Ruin of the Roman Empire, and/or Ward-Perkins' The Fall of Rome and the End of Civilization, but my knowledge on English-language literature on the topic is rather scant.
It's not about the Roman Empire but one book that rivals Gibbon in scope, depth and originality is The Mediterranean and the Mediterranean World in the Age of Philip II, by Fernand Braudel