The sight of Spike so bright-eyed and eager captured my smile in an instant. You did a beautiful job bringing out his body form, bright smile, and a very natural relaxed pose as he starts on what may be a very long letter. (It's on your letterhead so I assume you gave him a few scrolls because they were out. ) Great job with the foreshortening and especially how the shading/highlighting gives him so much more depth along with the smaller details in his body. The stack is a great makeshift stool (one I'm sure he's more than used to), and though it's canon the quill and ink still make it more charming.
Then I thoroughly enjoyed the time-lapse video of the drawing process. It's great how quickly the idea and structure came together, but there were a few new twists. The Celestia's crest bookmark would have been a neat touch, borrowing a personal book of hers, but then again the princess would probably appreciate neither the floor nor the dragon tushie being so close to contact with one of her personal favorites. Good thing Twilight is such a messy stacker. (That's how I work, but in most cases I'd have to put everything away every night and just note which page(s) of each I was using.)
Anyway, I also have to admit that he seems more handsome than usual, rather fetching actually.
Heh. I love this style for Spike. The actual spikes on his head being slicked backwards is a really good look for him. It makes him look more boyish. I love the expression, too, and the way his eyes seem to glow.