Why does Montag turn to books when he is unhappy?the book he possession is the last of its kind in his part of the world?

Specifically, in Fahrenheit 451, Montag turns to books because he is unhappy.  He wants some of what Clarisse has.

Montag is smart enough to see that television is simple-minded, that his marriage is without love, that he is incapable of looking at nature and finding something of value in it.  He is smart enough to know that something is wrong with a society in which technicians that pump people's stomachs after overdoses of sleeping pills, do it so often that they are nonchalant about it. 

Montag simply wants more.  Clarisse and her family show him what conversation can be like.  She shows him the interest in nature and love that she has.  And he wants the same things.

He turns to books in the hope that they can change his existence.  And, eventually, they do.