The Phoenix Unchained by Mercedes Lackey and James Mallory.

Harrier and Tiercel have grown up as best friends, Harrier always saving Tiercel from trouble.  Tiercel finds a spellbook and attempts to cast an easy spell.  Thereafter he has horrible nightmares.  Tiercel and Harrier travel to a city to try and find help.

Towards the end of this book, I realized this was book 1 in a second trilogy, of which I had started book 1 in trilogy 1.  Though I finished this book, I couldn’t finish Book 1 of Series 1, so I doubt I’ll try this series again.