The Hidden City — David Bowles

When Carol and Johnny learn of the Ollamat, an ancient stone that can channel savage magic, they convince their parents to take them to the cloud forests of Oaxaca. With Pingo's help, they search for the legendary city where it has been protected for a thousand years. But the twins aren't the only ones hunting for the Ollamat. After it is stolen, they must travel through an emerald mirror into the beautiful yet dangerous Tlalocan: the paradise of the rain god. To retrieve the stone, they must face talking apes and forest elementals, rock worms and vicious elves, demons of lightning and something even more unexpected: the souls of people they have watched die. As always, they are aided by allies old and new, though nothing can quite prepare them for the biggest foe of all - a member of their very family.