Why do you think Montag plants some of the books in a fellow fireman's house?

If you remember, this was Montag's original plan for how
to fight back against the society in which he was living.  So I think that he does this
as he runs away as a way of trying to fight back against the
society.

The idea behind Montag's plan was that he wanted
to undermine the reputation of the government.  He wanted to make people think that
there was something wrong with the people who were enforcing the rules.  So what better
way to do that than to put books in firefighters'
houses?

By planting the books there, he could make the
firefighters look bad and he could (he hoped) undermine the society a little bit in that
way.