Throughout the course of the book, he states several reasons. It is obvious that he is a well-read man; he has read books, plenty of them, and knows them well enough to quote them at Montag in rapid succession right before they are called to burn Montag's house. So, at one point in time, it is hinted that he loved books enough to read them and learn them well enough to process their meanings. But, he seems to have turned against them. Here are the various reasons he gives, in different parts of the books. After his history lesson to Montag, he states, "the books say nothing! Nothing you can teach or believe." He goes on to clarify that they are just a bunch of people who think that they are smart, trying to outsmart each other. Later, at the fire station, he says,
"What traitors books can be! You think they're backing you up, and they turn on you. Others can use them too, and there you are, lost in the middle."
So, he thinks that books give you great, profound thoughts-until another person comes along with his own thoughts from a different book and argues against you. They aren't things that you can rely on and trust. Later, after Montag's house burns down, he says,
"Give a man a few lines of verse and he thinks he's Lord of all Creation. You think you can walk on water with your books. Well, the world can get by just fine without them."
He thinks that books make you foolishly confident, they make you want to go save the world, to change everything. But, he says that doesn't work; it just brings heartache and trouble.
From the references in the books, it seems to hint at Beatty having been greatly inspired by books at one point, but then experienced some sort of tragedy or crisis, eventually deciding that they did no good, and so turned bitterly against them. I hope that those examples help; good luck!