I think this is one of the best things I've seen on deviantart in a long while. I love the colors, the textures, and just about everything else. The style you illustrated Hermione in is very different from every other piece of Harry Potter fan art I've seen. I really like the expression on her face, and at first glance it looks like her eyes are closed, but taking another look, it just looks like she's studied so much that she's getting drowsy. Crookshanks even looks like he's a bit worried about her.
I also like the contrast between the warm colors indoors as compared to the really cool blues outdoors; that really emphasizes that she's safe and sound for the evening. It's also worth mentioning that the moonlight on her hair looks great.
I also really like the details on the book she's studying, logos and artwork. Also having Hagrid's hut, with some smoke in the background, is another nice touch.
I wouldn't really call this a sketch because it looks very complete. I really like this one. Very nice job. I hope you didn't mind my interpretations of what's going on in the image.
Wow, this is gorgeous. And adorable! I can never get enough of snooty, bookworm Hermione like she is for the larger part of the series-you just see her glamorized so much. I love Hermione's hair, the detail in her and Crookshanks' faces and the book, and the watercolor is so beautiful.