Um, okay... Geez, I don't know what to say in my first critique, but I guess I can start with the picture in general. Since I don't exactly know how to draw on the computer, this is really good in my standards. I just love how cute everything is in this, it also makes me want to know what story Celestia is reading to Twilight. And Spike is all curled up next to her on the bed, I'll say this: SO ADORABLE!!! X333 Yeah, I have NO idea what to write in a critique, so I hope this'll do for now.
This is an absolutely adorable picture, and you've done a great job of capturing the idea for the scene. The colors have a very nice blend to them, and all the characters' expressions seem perfect.
A couple things I might recommend: 1. I might not use an overlay effect for Celestia's magic. The way it blends with the lighting just make the book look like it's kinda floating in midair. 2. Something about Spike's shape seems a little off. He looks more like a cat than a baby dragon, and I think the show portrays him to be a little chubbier.
However it seems to me, you had a very clear idea in your head of how you wanted this to look, and it seems like you took a lot of time and effort to work on this, and this is a beautiful, adorable piece. I particularly love the vibrant colors.