What is Montag reading just before the mechanical hound comes to the house?
I assume that you are talking about what happens at the very beginning of Part 2. There, Guy and Millie Montag are reading. Guy has pretty much made Millie do it. While they are reading, they hear a scratching at the door. At that point, they do not know that it is the hound, but they find that out later.
We are not told exactly what they are reading -- we are just given quotes. But we know from the quote that they are reading from the Life of Samuel Johnson a biography by James Boswell.