The crew ranks the books from A to J (J being the best) in terms of how much they want to play the game the book or wiki they've just read was based on. The system was thought up by Caldwell and Emily on Episode 4 - Bioshock: Rapture, Caldwell explaining that the J stands for "je sois".
It could be said that this is literally the same as a 1 to 10 rating system, but the person saying that would be wrong. This one uses letters.
"Aaaah, no thanks!"
"Aaaah, I don't think so!"
"AAAAH, don't make me fucking play!"
"Just play it, dummy!"