The Obsidian Mountain Trilogy

In the captivating fantasy world conjured by veteran author Mercedes Lackey and classical scholar James Mallory, there are three types of magic, each with its own rules, limits and variables. High Magick is a thing of complex rituals and specific spells.  In contrast, those who use Wild Magic—considered heresy in Armethalieh, the Golden City of the Bells—must bargain for what they need and may pay a high price for power.

Kellen Tavadon, son of Arch-Mage Lycaelon of Armethalieh, has been raised to believe that High Magick is the only true magic–and knows that he will never be the mage his father expects him to be.  One day, he finds a set of books about Wild Magic and soon finds himself declared an outlaw.  But the Wild Magic has a purpose for Kellen, who, along with a unicorn, an elf, and a dragon–all long-rumored to be only mythical creatures–must save the world.

Also in the world of the Obsidian Trilogy:
The Enduring Flame Trilogy
The Dragon Prophecy Trilogy