Bill Bryson's book The Mother Tongue - English and How it Got That Way is an in depth look at where English began and how it has changed through the years. It's an interesting read but it has so much information that I found it best read in small spurts.
England, being an island, has a history of invasions, by the Anglo Saxons, Vikings, Norman the Conqueror and the Danes. But English is a Germanic language (unlike the Romance Languages) and there is still a town, Angeln, in Germany where people speak similarly to the early, lost dialect of English. You only have to go a hundred miles or so in England to come across a completely different dialect and this may be because the Vikings, by treaty, took over the midlands of England and the Celts kept the south (Cornwall, Wales) and the north (Scotland). To add to the confusion, after the Norman Conquest, most of the aristocracy spoke French or Anglo-French. Mostly illiterate and uncultured, the people of England used the Runic Alphabet which can be found on tombstones and other stones.
In the early times England was a pagan country with the days of the week taken from gods, Tiw, Woden, And Thor (Tuesday, Wednesday, Thursday) Saturn, Sun, and Moon became Saturday, Sunday, Monday.
And then there's Cockney slang which anyone who has seen Pygmalion or My Fair Lady is familiar with.
Bill Bryson also talks about how the language changed in America, touching on the Gullah dialect of the Outer Banks (English combined with West African).