How do Granger and his group plan to preserve knowledge contained in books?
Each member of their commune chooses a book, then learns it by heart. After memorizing the book, the book is burned so that they will not be arrested by the authorities. As time goes on, the knowledge and stories are handed down verbally from one generation to another.
They replicate the text from books in a secret code that no one else knows. They will then teach other people how to read this code, and the knowledge contained in books will survive as more people learn the secret language.
Granger explains to Montag that their group has built a hidden bomb shelter where they stash books. They will seek refuge in the shelter when the war starts, and will have access to all their books and the knowledge within them.
They plan to murder all the firemen so that no more books can be burned. When Montag is aghast that they resort to violence, Granger explains that books are more important than individual humans.
The members of the commune plan to pose as firemen who pretend to burn books, but steal them instead. If they get enough people to pose as firemen, no more books will be burned.