Waking in Dreamland by Jody Lynn Nye

In a strange realm, Roan was the strangest of all -- because he always looked the same: normal. Dreamland was the place where sleepers in the "real" world went when they dreamed, and the permanent residents might change their form without warning. In dreams, there's nothing unusual about talking to a giant rabbit who suddenly turns into a living fire hydrant without missing a beat in the conversation. But Roan was always...Roan. And that was very bizarre.
But something sinister is going on in Dreamland. Their constantly changing world is created by the Seven Sleepers, and will continue to exist so long as at least one of the Sleepers is asleep and dreaming. Now someone is out to destroy Dreamland by eliminating them. And unless the nightmare plot is foiled, Dreamland and its inhabitants will vanish like a blown-out candle flame, bringing an end to all dreams....