I wouldn't say that reading a book multiple times is wrong. People learn in different ways. One way works for some and another way works for others. Anyway, finding the interesting parts of a book is a good tip, one of the axioms of learning is that you'll learn much faster if you are interested. Building interest on the book/subject by reading what we find interesting is an excellent way to learn. Of course you have to eventually read what you previously found uninteresting. Hopefully by then you'll have enough knowledge or curiosity to get thru it. What helps is to constantly ask questions as you go through the book and finding the answers will help. Generally speaking humans need to solve problems to survive. Our brain gets a hit of Dopamine every time we get an answer. So, by asking why as we read, we'll be rewarded by learning.

My tip is to make sure you understand the subject. By putting it in your own words. A problem with reading is that it's easy to fool ourselves into thinking that we know something when in reality we don't. We need to constantly test ourselves.