King Javan's Year by Katherine Kurtz

With young young King Alroy on his deathbed, the power-hungry Regents are plotting to keep the legitimate heir from ascending the thrown. But Prince Javan is a formidable opponent. And the Regents vow to stop at nothing to wrest back control of Gwynedd. To prevail against the Regents, to rule his kingdom justly, and even to live, Javan has to be strong. He has to be clever. And he has to be very lucky indeed...