There are three factors that are the key behind the happiness created by reading books. The first is quality, the second is leisure, and the third is the ability to act on what you learned from the first two.
To elaborate, quality means that a book has "texture...pores...features...[it] can go under the microscope" and show life's many different angles and meanings. It has meaning, depth, profundity, and thought-provoking ideas. It is rich in information, concepts, ideals, and ponderous messages. And when one ponders all that a book has to offer, one is really pondering all that life has to offer, and you are able to sort out what is truly meaningful, and what makes happiness possible. For leisure, it simply means that with a book, you have the "time to think" about the messages in a book. With other types of media, it is very difficult to think, because it is so overwhelming. A movie has lights, sights, sounds, music, everything, and it is so overwhelming it doesn't allow for thought. The movie does the thinking for you. It conveys the arguments it wants to convey and leaves you unable to argue. With a book, you can put it down, process the information, put your own spin on it, reject it or accept it. Then the third reason is that once you have the ideas, have had the leisure to ponder them, books prompt action. You are motivated to change the world, or the things around you.
I hope that helps!