how will Granger's group try to preserve civilaization?

They memorize books.

"Nothing. I thought I had part of the Book of Ecclesiastes and maybe a little of Revelation, but I haven't even that now."
"The Book of Ecclesiastes would be fine. Where was it?"
"Here," Montag touched his head.
"Ah," Granger smiled and nodded.
"What's wrong? Isn't that all right?" said Montag.
"Better than all right; perfect!" Granger turned to the Reverend. "Do we have a Book of Ecclesiastes?"
"One. A man named Harris of Youngstown."
"Montag." Granger took Montag's shoulder firmly. "Walk carefully. Guard your health. If anything should happen to Harris, you are the Book of Ecclesiastes. See how important you've become in the last minute!"
"But I've forgotten!"
"No, nothing's ever lost. We have ways to shake down your clinkers for you."
"But I've tried to remember!"
"Don't try. It'll come when we need it. All of us have photographic memories, but spend a lifetime learning how to block off the things that are really in there. Simmons here has worked on it for twenty years and now we've got the method down to where we can recall anything that's been read once. Would you like, some day, Montag, to read Plato's Republic?"
"Of course!"
"I am Plato's Republic. Like to read Marcus Aurelius? Mr. Simmons is Marcus."
"How do you do?" said Mr. Simmons.
"Hello," said Montag.
"I want you to meet Jonathan Swift, the author of that evil political book, Gulliver's Travels! And this other fellow is Charles Darwin, and-this one is Schopenhauer, and this one is Einstein, and this one here at my elbow is Mr. Albert Schweitzer, a very kind philosopher indeed. Here we all are, Montag. Aristophanes and Mahatma Gandhi and Gautama Buddha and Confucius and Thomas Love Peacock and Thomas Jefferson and Mr. Lincoln, if you please. We are also Matthew, Mark, Luke, and John."
Everyone laughed quietly.
"It can't be," said Montag.
"It is," replied Granger, smiling. " We're book-burners, too. We read the books and burnt them, afraid they'd be found. Micro-filming didn't pay off; we were always travelling, we didn't want to bury the film and come back later. Always the chance of discovery. Better to keep it in the old heads, where no one can see it or suspect it. We are all bits and pieces of history and literature and international law, Byron, Tom Paine, Machiavelli, or Christ, it's here. And the hour is late. And the war's begun. And we are out here, and the city is there, all wrapped up in its own coat of a thousand colours. What do you think, Montag?"
"I think I was blind trying to do things my way, planting books in firemen's houses and sending in alarms."
"You did what you had to do. Carried out on a national scale, it might have worked beautifully.