as a training... i read. That is all. I begun when i was 5. Never stopped. So i nearly always was like that. The more you read the more you train your brain to read. And your mind to understand how to deal with knowledge and information. Filter it, classify it, absorb it, apply it.
For non fiction, yes it happens. Lot of book sjust repeat the same thing over and over again. When you begin to read a chapter and can complete what will be said in the next 20 pages just from your understanding of the whole situation, reading it is a loss of time. And it would make me be bored and get down from "The Zone".
I keep track in my brain. I have the advantage of being able to always remember if i have read something by just looking at the backcover and the first lines. I still have to forget a book i read. I can not remmeber all the technicalities of course but far enough to know if i read it before or not.
I reread the books i really like or need when needed anyway. Mainly during vacations.
Secondly, it is the only way i can absorb information in a way that works. Talks, video, podcast, etc are too slow for me. It lacks a good throughput of information and meaning. Which means i tend to just drop or complete what the speaker is telling.
About applying knowledge : yes everyday, in my life. Once you hit a good amount of knowledge and have a nice way to filter it, think about it and deal with it, thngs become nice. Understanding a problem come faster. You can draw link between different situations or use ideas from other field into yours.
AS someone that read at least one book per day if not more since the age of 6, yes it is possible. I can read between 100 to 200 page per hour, depending of the book.
You reach a storage and money problem fast (Ebook are a savior nowadays). And you tend to have multiple books open at the same time.
How does it work? There are several strategy. First i read fast. Experience and training make you read really fast. Secondly, you get a grasp of how things works and what the wirter has to say. In a fiction book, it is not unusual for me to not read a chapter or two because i know what will happen inside.
Finally... Good writers helps. Good writers make reading a breeze and are faster to read. They present ides in concise and efficient way, that follow the flow of thinking.
I will take more question gladly if you have some :)
Scaleway seems to go in the right direction but still a bit of work needed