This is a great question to consider. Overtly at least, it is possible to argue that Montag gains very little through his fascination with books. After all, at the beginning of the novel, when he meets Clarisse for the very first time, it is clear that Montag is perfectly happy with his life and his job and is not aware that anything is wrong. However, the novel charts his increasing sense of dissatisfaction as he becomes more and more aware of how empty his life, his marriage, and his job, actually is. His search for meaning involves reading books, as he hopes in the pages of the books he manages to get hold of, to find the meaning that is so apparently absent from his life. In the end, when he flees the Mechanical Hound and floats on the river away from the city where he has lived, even though he has lost everything, he actually realises he has gained a massive amount:
He floated on his back when the valise filled and sank; the river was mild and leisurely, going away from the people who ate shadows for breakfast and steam for lunch and vapours for supper. The river was very real; it held him comfortably and gave him the time at last, the leisure, to consider this month, this year, and a lifetime of years. He listened to his heart slow. His thoughts stopped rushing with his blood.
What Montag gains is the time and ability to think at "leisure" and to consider his life and the meaning of his existence from a rational perspective. Yes, he has lost his wife and job and home, but what he has gained in exchange for these things has a value that is greater than anything. He gains a measure of peace and of calmness that gives him what he is looking for.