Wow, what a stunning pairing of art and story! I love the illustrations; they capture both characters so well, and they have an almost storybook quality to them. They really capture the quiet tenderness that grows between Snape and Firenze.
As for the story, it's a gem. Funny in parts, bittersweet in others, and always very--for lack of a better word--human. I love the calm, solid restfulness of Firenze and the solace that he gives to Snape, even if it's only temporary. Still, for me, the standout of the piece is the outstanding characterization of Snape, which is just spot-on: clenched up in ways he doesn't even understand, girded against expected disappointment with a ruthless self-honesty that's as admirably pragmatic as it is heartbreaking. There a strangely sweet innocence to his joy in discovering how good it can actually be to have one's feelings reciprocated.
A few times in his adult life he had been excited by some actual person in his circle, and he had found that a good, solid understanding that he was no one's fantasy himself kept him from being ridiculous.