I found this really interesting and evocative, and I enjoyed your way with words: before he knew it Snape had crossed the small space between them and retrieved the book with a swift, darting motion, the way a beak might plunge into the river to catch breakfast. I've never given much thought to Rabastan at all, but you've fleshed out his character in a way that is very intriguing to me. Like an earlier commenter, I found the last sentence a bit abrupt after falling into the sensuous mood you'd created.