Malorie Blackman 
Whizziwig and Whizziwig Returns Omnibus 

Whizziwig is a small, furry alien who has crash-landed on Earth and ended up on top of Ben’s wardrobe. Once Ben has recovered from the shock of this unexpected arrival he’s delighted to learn that Whizziwig is a 'wish-giver’ able to turn wishes into reality. But it’s not as straightforward as it sounds. The wishes have to be made for someone else, and by accident, so it isn’t long before some very strange, and very funny, things start happening.