Elf Girl and Raven Boy: Dread Desert: Book 4 by Marcus Sedgwick
Elf Girl and Raven Boy get hot and bothered in the desert as they come a few steps (with sand in their shoes) closer to finding the Singing Sword and the Tears of the Moon. They bump into a grumpy camel, a grumpier genie, Sultanas, Sandpeople and scorpions as they try to save the world from the Goblin King.