The writers are mainly professional teachers or native speakers, but many amateurs also write "How to study English" books. Amateur authors give us motivation for learning English. A major baseball player Hasegawa or succor player Kawashima gave us that. That's really good for learners' enthusiasms. It might be worth only for that at most.
Remember, there's no easy way to learn English, so if you seek a good result for your ability of English, you should follow professionals' methods. This professional doesn't mean earning money from English, but the attitude for English. Don't waste your time.