A very lightweight and simple way of improving retention is to re-expose yourself to the ideas. If you're reminded of a book, go flip through it again and read whatever bit strikes you at the moment. Just enough to satiate your interest, but not so much that you feel like you have to re-read the whole thing again. I like this because it's purely interest-driven. The books that plant the most seeds in your mind will draw you back the most, and consequently get the most re-read, and the get the most retention.

Just as you can remember the words to a song better when you hear the tune, you'll find that just reading a few pages of a book can reactiviate your recollection of the rest of the contents, you find that you remember much more than you thought you did. Another nice thing is that by re-exposing yourself to things you read before, you can connect them to things that you've read or ideas you've had since then. Connections that you couldn't have made the first time start forming. This, too, helps retention. It also feels good.

Not all books are great for this. Pop science books, as entertaining and informative as they can be, can frequently be summarized on a napkin without much loss. They tend to have one big idea and lots of details. The ones that work best are books packed with ideas, the ones that you finish on first reading knowing that you haven't fully wrapped your head around.

Incidentally, all of the books on Kay's list that I have read happen to be of this kind. I think that bodes well for the rest.