Gilbert Morris 
Spell of the Crystal Chair [EPUB ebook] 

The Dark Lord has been busy, and once again Goel is sending the Seven Sleepers to spoil his plans. This time Josh and his friends are off to Whiteland, a place of sled dogs and igloos, polar bears, and seals. The Sleepers soon learn that Whiteland is also the home of the dreaded ice wraiths, the warlike Yanti people, and a crafty dark-hearted wizard. Then they hear the really scary news: their job is to destroy a special chair deep inside the wizard’s palace. But what about the fearsome creature that guards the Crystal Chair? And what about the wizard himself? See what happens when the Sleepers obey Goel, the good leader who always gives his friends just what they need.