Granger and his clan have a very unique way of rebelling, and of reading books. Typically, to read a book, and to always have it with you to use as a reference, you have to carry the book around, and have the book with you. That way, if you ever need it, you can use it. That way, as Granger and his clan start their new society, they can bring all of those books with them to lay the foundations with. However, in their society (before it was wiped away with the bombs), if you were caught with books, your house was burned, and you were taken away. You can't do much good with books and a house that has been torched, and being in jail. So, Granger and his friends have adopted a new strategy: as soon as they encounter a book, they memorize it. That way, they can get rid of the book, but it stays with them forever, and they will always have access to it. Also, if ever stopped by the authorities, there is no evidence of book-reading because it is in their heads; they don't have the hard copies to be found anywhere.
Now, memorizing books is very hard, so they developed a strategy, practiced and practiced, and made it a discipline and an art form. Granger states to Montag that they
"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."
This strategy enables them to absorb more books, remember them longer, and have them whenever they need them. Pretty cool. I hope that helps!