Ellis in Imaginedland (or Prince of Whales), by Rocky Leplin
Far in the future, a teenaged girl named Ellis Antlers is the last person left on Topside, her side of the planet.
After meeting the Guardian of Imaginedland and falling through the Earth, Ellis finds that her Topside compatriots have been there first, and all have been brainwashed in the sinister land of Purplevania. With a walrus-mustachioed Anglo-Siberian named Borax Meerschaum, the Prince of Whales and Leader of the Overground; his shape-shifting Golden Retriever sled dogs; and a slightly goofy athlete named Ribs Mgumba, the Congo Kid, Ellis encounters many strange hazards on her mission to unbrainwash her friends.
Near the end of Ellis in Imaginedland (or Prince of Whales), Ellis is unsure if she’s back in the real world, or still in Imaginedland. And isn’t this something everyone can relate to? After all, it’s your imagination that takes you on the wildest ride.