"Midnight on Saturday! Oh no oh no -- I've just remembered -- Charlie's letter was in that book Malfoy took, he's going to know we're getting rid of Norbert."
-- Ron Weasley (PS14)
Ron’s hand has swollen to twice normal size and turns green after Norbert bites it. He spends the next few days in the Hospital Wing recovering. Fortunately, Madam Pomfrey doesn’t ask too many questions. During that time, Malfoy comes to visit Ron on the pretense of needing to borrow one of Ron’s school books. He taunts Ron about Norbert, threatening to tell the teachers about the illegal dragon, then borrows the book for show. As it transpires, Ron had put the letter from Charlie in that book and from this, the trio knows that Draco is on to them about bringing Norbert to the Astronomy Tower at midnight on Saturday (PS14).
This is one of Rowling's weaker plot moments. The idea that Draco would pretend to visit Ron in the Hospital Wing just to taunt him on the pretext of borrowing a book is strange enough, but then for Ron to give Draco that one particular book that just happened to have the letter from Charlie in it ... well, clearly Rowling needed some way to get Draco on the scene to tattle on the dragon release on Saturday night and from that to set up the Forbidden Forest adventure to come. But it all seems very contrived. --SVA