Thats a very interesting chapter for me, and I am not ready to understand it completely….
- I believe Snape already had the idea that Harry is in possession of his old book, when he heard him praised for his abilities in Potions by Slughorn - he is sure of it when Harry used the Sectumsempra, what Snape invented and wrote down in his own book
“Apparently I underestimated you, Potter” he said quietly “Who would have thought you knew such Dark Magic? Who taught you that spell?”
Snape exactly knows this…..but remains in cover. He knows Harry is lying to him about that book.
When he asks Harry to bring all his school books, he is looking at the Potions book intentionally in the end….. He knows.
I believe the detentions Harry is having with Snape following that episode are pretty kind.
It was not Harrys intention to hurt Draco that bad and he is the Chosen One and because of that there is no way to expel him……..
Snape is indeed furious he used that spell of his own, but I believe much more because of Harry endangered Dracos life and also Snapes life with that unbreakable vow on him. He tries to protect Draco all the time with all his might and then there is Harry messing up everything even more.
I believe those detentions are not only to punish Harry but also to have Harry out of the way those hours without the risk having him follow Draco those times on his heels…..