How did the ever-increasing speed of society affect books according to Beatty? What's the most important thing in society?

I assume you are talking about the things that Beatty says
when he visits Montag in his home.  This is the time when Montag has stayed home sick
from work.

According to Beatty, the increasing speed of
society made it so that books withered away.  People just wanted shorter forms of
entertainment so books got boiled down to digests and summaries and then eventually
disappeared altogether.

As to what the most important thing
is, I would say that it is happiness.  That is what Beatty says that they are
protecting.  You might say, though, that it is mindlessness -- just filling your head
with this pointless entertainment so you don't have to think.