An expression which originated during a drunken late-nite discussion of the origins of expressions. It means, "I see what you're saying, where you're going with that."
When you finish a really good book or tv show or whatever and you feel all empty and achy and sad. usually these feelings are followed by large consumptions of food, sleeping all day and just being all around depressed.
When someone goes book, or book mode, they have messed up. They were unintelligent , made a petty mistake, or were oblivious to something obvious. Being book is nothing to be proud of, so now it has been made into a meme