Now You See It by Vivian Vande Velde
When Wendy, who is blind without her glasses, finds a pair of ugly mirrored sunglasses that fit her prescription exactly, she has no idea that the world as she knows it will turn upside-down and even a little backwards. Not only does her new find allow her to see with 20/20 vision, it also lets her see everything else.... like the cheerleader who is really an old crone, the geek who is an elf, and the little blue men who steal what you're looking for and eat freshly laundered socks.
All this is a bit much for Wendy, especially when the geek/elf turns out to be a prince and she is responsible for his kidnapping. With the help of her grandmother who travels through time to aid her, she has to rescue him, avoid getting killed by the dragon, and find out how good and brave a person she really can be.