The Tower of the Elf King, Book 9 by Tony Abbott

A hidden door. A magical staircase. Discover the world of Droon! Mysteriously, Lord Sparr has disappeared. But there are still many strange happenings in Droon. For one thing, somebody has pillaged the village of the Lumpies, and Eric, Neal and Julie are asked to find out who it is. With Keeah, they begin to investigate. At first they find that the thief is a monstrous, scaly lizard called Gryndal. But after a hilarious "battle" with this beast, they find that Gryndal is really a hog-elf in disguise! Gryndal has spent many years under the spell of Lord Sparr. With Sparr out of the picture, Gryndal is finally free. But now he needs to get home - and a creature of his stature can't risk the dangerous journey on foot. With the help of some mercenary Ninns, Gryndal's been forging a tower, a patchwork of metals, to meet some elusive Soarwings as they fly above the clouds - and, hopefully, hitch a ride home. On the top of the tower, though, Gryndal is amassing a pile of glittering treasures to attract the colossal birds. The kids want to help him. Gryndal is a sympathetic character - and he also has some key information about Keeah's mother. But is it right to take his side if he's stealing others' treasures to get where he wants to go?