Hermione is studying alone in the Head Girl's dormitory when McGonagall finds her. She is surprised when the portrait hole opens; Ginny isn't due back from Quidditch practice for hours yet. Plenty of time yet to re familiarize herself with the properties of engorgement charms. McGonagall lets herself in through the portrait hole.
"Miss Granger," she asks, "Are you alone?"
"Yes, Headmistress," Hermione says putting her work down and turning to face McGonagall. Hermione finds it difficult to look at her.
"First of all," says the Headmistress, "You need to get a handle on your guilt. The fact that you still feel it means you aren't what you accuse yourself to be. Don't fall prey to such a blatant logical fallacy."
Hermione can't help but smile at this. Leave it to McGonagall to use logic to get her to stop feeling so guilty. And its working, just a little.
"Now," says the Headmistress when she sees her approach was successful. "I have something for you." McGonagall gives Hermione a book.
Hermione runs her fingers over the silver lettering on the cloth cover. It reads, A Study of the Phenomena of Bright and Dark Magicks, and the Energies that Produce Them, by RJ Proctor.
"You asked me, Miss Granger, what you could do for me by way of apology."
Hermione nods miserably.
"I want you to read this book. Then, I want you to read it again. And again. And again. Make it become a part of who you are. I can't make you dedicate your life to bright magic. But if you are going to walk the line between bright and dark, I can damn well make sure you gain mastery and deep understanding of the two. This book, combined with self reflection and ruthless honesty with yourself, can give you that."
Hermione closes her hands around the book reverently and says, "I will."