I adore this. From the moment Harry started ruminating on Snape's handwriting, I was smitten. And with each letter, with each twist to their mercurial relationship, this story soared higher up in the ranks to become one of my favorite Snarrys. The depth of their every interaction reminds me why it was I fell in love with Snarry so many years ago and why it is that I cannot ever seem to break free of this pairing.
Besides that, on a more technical note, I enjoyed Kingsley's lecture on investigative techniques and deductive reasoning. It's nice to see a side to Auror training that doesn't involve hunting people down or blowing things up. And I especially liked the purpose of that particular scene in broadening Harry's self-awareness as well as the conclusion it led him to. Harry really could do with lessons in reasoning, and I liked that it was addressed here, even peripherally.