Great Brain Robbery, Paperback/P. G. Bell

Suzy can't wait to return to the Union of Impossible Places on the Impossible Postal Express. But when she arrives, she overhears a dastardly plan to destroy Trollville from a shadowy and unexpected villain. Suzy and her friends must race from magical cloud-worlds to secret caverns to catch the culprit, before Trollville comes crashing down...