For anyone with a genuine interest in the history of English, I recommend "The Adventure of English," by Melvyn Bragg; it's actually very readable, even entertaining in places. He maintains that English has survived by basically being a thief, and pinching words from other languages, then making them its own. He also says that it was the Black Death which ensured the language survived, since all of the educated types, like monks, were either killed, due to living in close proximity to each other, or ran away. This left only native English-speakers available for court duties, etc., so the language continued.