Book index can be a great way to see what the author thinks about a specific subject. You can see the important theories, concepts, key authors…
Since I have no memory, bur still wants to quickly see what was really important in a book, I make little mind map of a concept.
For example with Al-Deen, H. S. N., & Hendricks, J. A. (2012). Social media : usage and impact. New York, NY: Lexington Books.
I have an image of this in my endnote, so if I need to refresh my memory, I can just look at it. It is also useful for the lit review. That book was centered around education, strategic communication and legal issues. So it’s not the place for me to look for information about social media and relationship maintenance. So 5minutes wasted doing the mindmap, can save me time later on when I have to go back and search in all of my references for something specific.