You can't find the interesting ideas and topics outside. They are in you. It has to come from your knowledge and experience. For some reason, while reading books, lots of thoughts surface. Sometimes I have to re-read pages because I will just continue reading while thinking about something else. You can write the thoughts down and if they are good, turn them into posts.

My guess is that since the reading is too slow compare to your thinking, it allows you to think deeply about things and gives those thoughts an opportunity to surface.

I think this only applies to non-fiction books though.