The reader is the one who brings ideas to books
Some smart books are really inspiring. They are absorbed quickly and with tremendous pleasure. Your head is filled with new ideas and deep thoughts. But not all smart books are like that. Not even all excellent, bright books have such an effect. It's not a piece of new information that makes a book so interesting. We are too resistive to tides of unprocessed thoughts. They are hard to digest.
Yet fresh ideas "from books" you create yourself. They are (and were before) floating in the head like in the air, unstructured, lucid, amorphous. A truly good book crystalizes them, as if an object is thrown in a bowl with pure steal freezing water, it will get covers in ice, and water will turn to ice. As a thick solution crystallizes upon the intervention of an outside item.
That's why when you reread great books, you always find something new that you did not see before. It is you, in your new state of mind, who is pushing the cart of organized thoughts.