After Montag has witnessed the woman choosing to burn alive with her own books, & has stolen a book himself, Beatty comes to his home to tell him the history of firefighters, and culture as it now existed. After explaining the "need" for forced equality, he tells Montag:
Books, so the damned snobbish critics said, were dishwater. No wonder books stopped selling,. the critics said. But the public, knowing what it wanted, spinning happily, let the comic books survive. And the three-dimensional sex magazines, of course. There you have it, Montag. It didn't come from the Government down. There was no dictum, no declaration, no censorship, to start with, no! Technology, mass exploitation, and minority pressure carried the trick, thank God. Today, thanks to them, you can stay happy all the time, you are allowed to read comics, the good old confessions, or trade journals.
So, the first reading item allowed were comic books. Apparently, they were considered less intellectual than actual literature. The second item were pornographic magazines. The third and fourth items are the "good old confessions" and the trade journals of various industries.