On what page of Fahrenheit 451 is the line, "Behind each if these books, theres a man. Thats what interests me"?

The quote that you are referring to starts on page 51 and continues on the top of 52.  I have the 50th anniversary edition of the book.

The quote comes from a conversation that Montag is having with his wife Mildred, when he has brought books into their home and he is trying to convince her of their importance.

"And for the first time I realized that a man was behind each one of the books.  A man had to think them up.  A man had to take a long time to put them down on paper." (Bradbury)