I don't have a link per se, or at least not to one that isn't written at a college level.
Just explain that there are different grammatical rules based on where the location, audience, and format. You can explain location in the context of America, Britain, Australia, etc. It's all "English" but with slightly different words and pronunciations.
English doesn't officially have formal and informal tense. Most other major language do however have formal/informal tense. Instead that gets conveyed through different speaking styles. She wouldn't talk to you or an authority figure the same way she would talk with a friend on the playground. The former is more detailed, slower, flatter annunciation, and built for literal clarity. The latter is based on brevity, nonverbal signals, a lot is communicated through annunciation, and it's built for faster communication.
Books, movies, tv shows, etc have to communicate differently to the audience. Some formats work better for convery information and details than others. I'm sure you can come up with your own examples. Books, often need to convey an informal conversation, and will use informal langue and accent pronunciations bewtween the quotes (often with incorrect spelling), to convey the context and environment surrounding the conversation. It may not be grammatically correct for a written format, but it is necessary to describe to the reader what is happening without distracting the reader with extra text.
