Disturbing is right!  You sure know how to manipulate the reader's emotions.  Of course, you prepared us for this in the last chapter, and, knowing Marietta, I should have expected a cruel, even sadistic, attack. But her twisted use of the words 'tough love' in the last chapter hinted that it would be a cold, clinical torment, that Marietta was not deriving any particular pleasure from it (like Pansy would).  That's what makes this character you've created so interesting.  She's not motivated by jealousy or sadism, or even power, so we keep wondering: what makes her tick? 

What really got me in the library scene was the little wounds: your use of Cho's friend Ian to turn on her (and his reaction afterwards) and Devin Smith, a seemingly kindly Gryffindor.  I found myself saying "Don't give him your wand, Cho!" just the way I would yell at the movie screen, "Don't go down into the cellar alone" to the next victim in a slasher film.

And the little side plots about Ron and Hermione, and Luna, and Harry's efforts to come to grips with fame and ego are, as always, thought-provoking and often hilarious.

And now Marietta has The Book.  I am dreading her next move.  But that's what's so good about this story.  We're always pondering the motives of the characters and wondering what will happen next.  You give us hints, but the outcome is always a little different than what we expect, and then you add another layer to the plot.

Creating momentum, that's one of the essentials for a effective story-teller, and you sure know how to do that.

She had left the window open waiting for an owl and had a few treats ready for its arrival.--So very Cho.

“You’re never cold, Eskimo,” she joked.--LOL.  And 'Dumbledork' made me smile, even though it was in a scary conversation.

Danielle and Marietta sound like Nazi Brownshirts.  Makes me shudder.  The venom and hate in the dialogue is so realistic.

You make a nice counterbalance of humor when Luna teases Cho about the library book.

What could Marietta's weapon be?  Hmmm--maybe Ghostbuster-style particle accelerator??   ; )