To understand the significance of Montag planting books one must put the novel in context. Books were censored because they promoted class differences between the well read and the not so well read. In addition, other forms of entertainment competed for people's attention. Montag's duty as a fireman under Chief Beatty - a well read man - was to burn books.
It was upon meeting Faber that both Montag and Faber questioned their cowardice and decided to print books and to plant them in firemen' s homes.This act represented the turning point in the novel and is fitting because it represents the embrace of knowledge which will bring about a change in society. There is the analogy of the merging of fire and water to produce wine which portends a miraculous transformation of society.
So the firemen who were the book burners would now become the knowledge lovers.
In Ray Bradbury's Fahrenheit 451, Montag speaks to Faber about how to combat the Government's practice of destroying homes, books and lives to keep people socially and intellectually anesthetized.
Faber and Montag are secretly discussing what they can do to bring about a change. Montag wants Faber's help, but the older man is fearful.
The only way I could possibly listen to you would be if somehow the fireman structure itself could be burnt. Now, if you suggest that we print extra books and arrange to have them hidden in firemen's houses all over the country, so that seeds of suspicion would be sown among these arsonists, bravo, I'd say!
Montag listens to Faber who (though Montag does not know it) is not really serious about the plan. However, the fireman who has been so changed since meeting Clarisse and then witnessing the burning at 11 No. Elm Street tries to make sense of what Faber has said. He repeats the suggestion back to Faber who, in turn, begins to look at Montag through new eyes. Montag is completely serious.
The more they speak of it, the more excited Faber becomes because he can see the irony—and the poetic justice—of such a plan.
It's an insidious plan, if I do say so myself...To see the firehouses burn across the land, destroyed as hotbeds of treason. The salamander devours his tail!
Note the irony: that firemen's houses would burn. Notice also Faber's use of the pun "hotbeds" while referring to the starters of the fires, now being implicated as criminals in the very crimes they were hired to stop.
Montag can provide critical information with which to move the plan along. He has access to a list of firemen's houses all over. He believes that if some kind of underground movement could be organized, the movement could use these addresses to plant books in the homes of firemen, in essence destroying the credibility of firemen as well as removing their participation in the any further destruction of books—with the destruction of their homes and lives (loss of job, freedom, etc.).
This is fitting for two reasons. There is irony in that firemen who have destroyed so many homes and lives because of the illegal possession of books would become the Government's most recent victims. In addition, it would rock the foundation of a society that has accepted the absence of books. One could hope that such an act would bring about some kind of response from a society made up primarily of those who have forgotten what it is like to think on their own.
It is also fitting in that Montag is a fireman. In a job where so recently he had delighted in watching books burn, he will destroy the system of firemen that destroy book. He has now broken free of his mindless existence. Instead, he has begun to engage in independent and original thought, something for which Faber praises him. It is fitting that one of the firemen's own members would be someone so central in bringing change to society, first by dealing the community of firemen a deadly blow against its power to hold society prisoner; and second, to become a leader to others when he has so recently been a captive himself.