This may sound like a dumb question, but how much do you think you retain and take with you from each book through out a year?

I know it will depend on the book, if it's a book to entertain or a technical book... but sometimes I get the feeling that so little is retained it's almost unproductive in the long term simply because you will forget a lot of it.

I have books that stuck with me, that changed my perception, but those are a really small fraction, and probably had to re-read them to get some details if I wanted to talk about that book to someone for example.

Do you take notes? If so, do you revisit those notes?