In Fahrenheit 451, why does Montag plant the books in Black's house?
Montag leaves the book at Black's house as Montag is on the run after having killed Beatty. Earlier, when Montag had gone to Faber's house to talk to Faber about books and how to bring down the anti-book society, Montag's plan was to plant books in the homes of firemen, thus bringing about the end of the society from within (a salamander devouring its own tail). Montag believed that if those who were in charge of enforcing the law were found in violation of the law, then the laws would topple. By the time Montag plants the book at Black's house, though, this plan is obviously not going to come about, so Montag's planting of the book at the home of a fellow fireman is more of a symbolic gesture at this point. The society is coming apart from within but not because of books being found in the homes of firemen - the people themselves, through their own lack of knowledge and intelligence, are bringing down the society in the form of the war that is about to open up. This is what Bradbury is telling the reader, that reading and being aware are essential to maintaining a society.
After Montag kills Captain Beatty, he narrowly escapes the Mechanical Hound as he is being chased by the authorities. Montag then hides in an alley and looks up to see the helicopters in pursuit. He realizes that the authorities are closing in on him and has the idea of hiding several books in Black's home. Montag sneaks into Black's house and plants the books inside the kitchen. Montag then walks across the street and calls in an alarm on Black's home. The helicopters and firemen immediately head towards Black's house as Montag slips away. The reason Montag planted the books in Black's home was to distract the authorities who were pursuing him. Montag's plan gave him enough time to escape to Faber's house before he fled the city.