There are so many ways to do this and a ton of aspects to it. Getting the most out of a nonfiction book is a process and not one that many people follow. Having read hundreds of nonfiction books and having gone through Mortimor Adler’s book of the same name, I have a set method to share that will give you the best chance of retaining and copturing the bulk of a book.
I used to just pick up a book, start at the beginning, and read it to the end. I tried not to bend any pages and always used a bookmark to make sure the book still looked brand new when I was finished. I had an unwarranted fear of ruining my books because I held them in such high regard.