First of all, having a good understanding of basic grammar rules is essential for English learners when it comes to doing extensive reading
Ok, you can read books extensively. I am not stopping you doing it, but wait a minute. How could you enjoy reading books and develop your English skills without having a solid understanding of basic grammar rules such as some basic sentence structures and tense, etc. Someone argues that you can learn grammar rules as you read books extensively, which is somehow understandable. But, I reckon that it will take so much time to bridge the gap between correct grammar rules and your misperception or misunderstanding of them. In most cases, you are going to give up reading books, because you feel it's difficult to read and understand the main points of what the book is about due to lack of grammar knowledge. If you had already learnt it properly before, you would have understood and it would have been much easier for you to read books. I know it's dead simple, but not many English learners do learn and master basic grammar first prior to doing extensive reading. Having a good understanding basic grammar rules makes your life so much easier.
Second, you cannot really get the hang of what a book is about if your understanding of grammar rules, especially some basic sentence structures is blurry
If you want to read books extensively, then you should have at least some basic sentence structures and rules such as SV, SVC, SVO, SVOC, SVOO, etc. It's going to frustrate you if you have to read S(ubject) and then O(bject), and go backward towards V(erb), which is nightmare for extensive reading. It's the best way for us to read SVO, not SOV. Extensive reading is quite a useful way if you can follow this way and know basic vocabularies without having a heavy reliance upon an English dictionary.
Third, having the ability to peruse English sentences is a prerequisite for English learners to do extensive reading
Personally speaking, doing extensive reading is nonsensical if you can hardly peruse basic-structured English sentences. I've been seen someone's reading books extensively, but said she or he couldn't understand the contents themselves due to lack of ability to peruse and precisely understand. I am not saying your comprehension must be 100%, which is not required and even not really possible unless the topic and the area of the expertise is within your realm. I have to underscore, however, the importance of having a solid ability to peruse various basic English sentences. It's not a big deal if you have done one grammar text, which is more than enough to be able to cope with this situation. For me, I bought one grammar text, and went through it a couple times. After a few months, I started perusing books, magazines and newspapers. I didn't have much difficulty in understanding all basic English sentences. You see how simple I am typing up things I want to convey to my readers in English. To this end, you need to go through intensive reading until you could confidently read and understand pretty much every sentence structure.
Doing extensive reading won't be really helpful if you're seriously eager to develop your comprehensive English skills like raising the wage floor
Reading books extensively enormously helps expand your general knowledge, but not English skills themselves. Thus, extensive reading is beneficial for those who are already sort of good at English and want to enjoy absorbing new words, knowledge and so on and so forth. Instead of reading extensively, what should be doing is to read books intensively, which means that you read and understand sentence structures as you peruse them. Extensive reading is usually for advanced English learners due to the nature of garnering new knowledge and words, but not learning grammar, which must be done prior to doing extensive reading.
That's all for today! Thanks for reading.
Have a good one!