Books can be great teachers, or they can just be mental masturbation. It's fun to read a self improvement book, but if your forgetting what you read and not actually improving yourself, you've missed the point.
To get rid of forgetting what you read and engaging in mental masturbation, I've started trying to condense each major part of the book into a 1-2 page summary of the key bits.
Then I summarize all those into a 1-2 page summery of the whole book.
This is awesomely useful because, not only can I come back and essentially re-read the book in 5 minutes, but I can share this with friends and mentors. It's a great gift because you're giving them knowledge you think they should have (like when you recommend they read a book), and you are also saving them hours of reading time.
If the book is particularly potent in how much it's teaching me, I'll read each section fully first and then immediately go back to re-read it and make my summary. This way, you have a better handle on which bits are significant.
P.S. Also, trying to stick to only books that high level mentors or a lot of your valuable friends have been recommending to you cut's down on how much low quality you have to sift through.
Hey, help me out. Like this post if it was useful at all, so I know if I should keep writing, and who I should write to.