depends on the book? I've read lots of books where it turned out the author had effectively one idea, it should have been 1 chapter, but they turned it into a book with 24 chapters of filler.
Of course this isn't always true but it's true quite often.
Take one random example - Spark: The Revolutionary Science of Exercise and the Brain.
The idea is in the title. You don't need to read more than that to benefit from the idea. But all the different varieties of benefit and pathways and studies the author sites are still valuable.
Range: Why Generalists Triumph in a Specialized World
idea is also in the title,and it displays so many different scenarios of people engaging with specialized fields and interacting with them in ways that relate to their past experiences.
Productivity hacks and pop psychology are not what we're talking about here. We're talking about interesting works of non-fiction. And if it's fiction, and you think that there is "one idea" and you can skip the rest, I don't know what to tell you.