/ New English 'teacher' in Poland (I have no qualifications).
It's interesting that you mention coursebooks as I firmly believe that they are one of the great evils in ELT today.
A number of years ago I decided the purpose of most monolingual English textbooks isn't to teach at all. The purpose is pathologize the student by diagnosing them ("you're at the upper pre-intermediate level!") and getting them into a graded system where finishing one level doesn't prepare you to do anything but take a course at the next higher level. That is, the purpose is to keep students paying money for as long as possible.
I've yet to find any evidence to make me change my mind.
That said, there are some good Polish-produced textbooks that a student can learn from. But the monolingual books from the UK seem like all filling and no content.
Fortunately I only deal with very advanced uni students (not technically English specialists but close) and can avoid using the mandated textbooks over 90% of the time.
We all went to school and took English(people from English speaking countries) for several years, learning grammar, spelling, writing, poetry, literary works, I think it may be enough to teach English if you understand what was taught in school.
The problem is that the language curriculum in English speaking countries is awful. The traditional grammar model for English is terrible and wrong (since it's based on Latin) and can't be operationalized but for various reasons schools can't use anything else. The last I knew British schools had mostly dropped any mention of grammar for students.
The grammar that ESL students get (when grammar is covered) is far superior to what native speakers get (mostly a bunch of zombie rules that have never been valid).
Also, most everyday explanations that untrained English speakers give are wrong.
The "simple present" is not a present tense at all (it's a habitual and gnomic tense) and the "present continuous" does not necessarily describe something happening now (the single most relevant parameter is 'not over yet').