In Fahrenheit 451, what starts Montag on his interest in books?
As a fireman, tasked with burning any books discovered in society, Montag starts with a very specific view on books. After he meets Clarisse, and after his wife Mildred overdoses on sleeping pills (it is unclear if she did it on purpose or not), he begins to really question the societal ban on books. When burning an illegal library, with his thoughts disturbed by his recent experiences, he accidentally reads a line from a book:
In all the rush and fervour, Montag had only an instant to read a line, but it blazed in his mind for the next minute as if stamped there with fiery steel. "Time has fallen asleep in the afternoon sunshine." He dropped the book. Immediately, another fell into his arms.
(Bradbury, Fahrenheit 451, Google Books)
While his changing perspective begins when he meets Clarisse, his direct interest in books, the very thing he is charged with destroying, starts in this scene. The power of written words, especially of images and ideas instead of the meaningless platitudes from television, begins his new interest in reading. He steals a book, almost without realizing it, and from then on his personality changes from a blind member of society to a free-thinking individual.