'Gunk Aliens: The Elephant's Trump' by Jonny Moon.
They don't want our oil, our water or even our brussels sprouts - they just want our snot!
Jack, Oscar, Ruby and their robotic dog Snivel have defeated one of the gunk aliens, but there are still 3 more to be captured. And that's bad news when the alien you're looking for likes nothing more than the smell of farts...