Good advice for language teachers and learners
The first thing to say about this is that the English ‘acquired’ by its authors is itself far from flawless; however, the book counsels us against correcting people’s mistakes so I’ll say no more about that. Otherwise, its practical conclusions could be summarised in a few paragraphs and are mostly a matter of common sense (notwithstanding the fact that they have largely been ignored by education systems in the English-speaking world, which is notoriously terrible at learning other languages): create a positive atmosphere, don’t worry too much about formal grammar, as far as possible use natural language situations and concrete, ostensible ‘referants’, allow learners to get used to hearing a language before expecting them to speak it, etc. The rest of it, though interesting in places, would be needed only by serious students of linguistics.
It’s not really rocket science: language teachers (and for that matter learners) should, as far as lies in their power, re-create the conditions experienced by a child learning their mother tongue. The nearer they come to doing that, the greater the chances of achieving fluency.