The game changer for me was actually learning how to read a book. I no longer approach books the same way after having read How to Read a Book by Mortimer Adler [1]. Once you have a method down, you can go through books rather quickly and retain knowledge. My biggest epiphany is that you don't need to read a book in its entirety to know all that it has to say.
[1] https://www.amazon.com/How-Read-Book-Intelligent-Touchstone/...