When you can write a book about a subject and explain it to other people, then you're a subject matter expert. Isn't this the point of narration?
I think fitness is a good example of something a person can become fairly knowledgeable about just by reading (nutrition is another one of these things) because as we read we apply it to our lives and relate it to our own experiences. We make those connections. We talk about what we're learning to other people. We also already bring enough background knowledge to it to be able to grasp enough vocabulary not to me totally stumped-- I know what muscles are, and legs are, so it's easy for me to remember the quadriceps and the hamstrings, the gluteus maximus and minimus, etc.
Reading is valuable, but watching lectures-- in person or on the screen-- is another mode of learning that is ubiquitous, easy, and can often augment books. But ultimately, nothing replaces a conversation with another person, and that's one thing I hope I can emphasize to my kids-- I always thought if you were smart, you could figure out everything by turning to books, but being able to discuss things and ask questions is so essential to understanding-- you have to be able to comprehend a lot to ask questions.
(In high school, I used to read books aloud in French because I liked speaking French and practicing my accent. I didn't understand 50% of what I'm reading and am no more an expert in French literature than if I had spent the time sleeping with a cassette player under my pillow.)