Montag decides to read books. The previous answer is an excellent explanation of the rationale behind Montag's decision.
He had been exploring his feelings of unhappiness and emptiness. One of the major themes in this book is alienation and lonliness caused by living in a totally conformist society. Montag confronts unhappiness head on by finding his wife unconscious from an overdose of sleeping pills.
His wife's recent suicide attempt and Clarisse's death by a speeding car only pushed him closer to finding out more about books. But after Mrs. Blake's immolation, he began to wonder if the answers to his unhappiness were not in the books they were burning. He takes a book from Mrs. Blake's fire to add to his collection of stolen books.
He determines that he will read these books and maybe even quit his job as a fireman. He wants to escape from the "constant contact" of television, radio, noise, and the friends who constantly come over for meaningless chatter about the war.
First of all, he decides to not go to work that night, and maybe never again. He asks Mildred, "how would it be if, well, maybe, I quite my job awhile?" He is feeling ill from what happened, his world is turned upside down, and he just wants to be able to stay home and process it all. Secondly, I think that he decides to take the step forward and actually start reading the books that he has hidden. At Mrs. Blake's house, he takes another book, and when he gets home that night, it states, as he hold the book, "His hands were ravenous. And his eyes were beginning to feel hunger." So, he was feeling that need, that burning desire to read. Then, after Beatty leaves, Montag gets out the books and really sits down to read. He tries to read with Mildred, but she is whining and upset, so that doesn't work too well. He ends up at Faber's where he sets up a plan to frame firemen houses to burn. So on the simplest level, after the Blake fire, he decides to not go to work that night, and to start reading. But those decisions start a chain reaction that set him on the road to open rebellion, and to enlightenment.