[BeetleB]

> So why every reading guide I've read talks about how to spend even more time on each book increasing investment, instead of how to read from book just the most valuable key points in no time?

The classic "How to Read a Book" kind of recommends this. They recommend skimming the book really quickly (e.g. one or two days) to get an idea of what it's about and the big picture - skipping anything you don't understand. And only then should you ponder whether it's worth a proper detailed read. They imply that most books will not be.