Reddit reviews OK: The Improbable Story of America's Greatest Word

This is total bullshit by the way. OK was coined in 1839 but a newspaper editor.
This book goes into more details.

This is the most accurate etymology I've heard. There was recently a book written about it.

There is actually an entire book written about the word OK. (I always use OK, for whatever reason.)

Here are two interesting articles about the book.