Alabaster - Achillios Chronicles, #1

Taryn wasn't expecting his younger brother's Test Day to be an ordinary day, but he had no idea it would turn out as it did. Rather than beginning a humble apprenticeship to a master instrument maker, Taryn found himself thrust into the strange and mysterious world of the Tower of Alabaster, suddenly charged with the city's welfare and protection. The Tower is astonishing, and its Servants are capable of controlling the weather, moving the earth itself, and other wondrous tasks.
He finds himself at the center of a complex inter-city political balance, with Alabaster delicately positioned on the scales of an all-out regional war. As Taryn tries to find his new place in life, he discovers intrigue, centuries-old mysteries and misunderstandings, and the inner strength that he will need in order to save the city and all who live within.