Dylan's book Farmyard Tales Christmas includes a picture of Poppy's letter to Santa. Poppy's writing is clearly shown within the letter, so Dylan has learned what 'letters' are. Now he calls the text in books 'letters'. (At times he distinguishes 'numbers', too.) In one of Dylan's library books, a mischievous character steals the letter O. Consequently Dylan now spots the letter O in books (and on his bottle, and on Daddy's sweatshirt, etc.) and says 'letter O' and makes the ooohhh sound. Obviously Dylan is a literary genius, in Daddy's view.