Since this has gone 36 hours without an answer I'll expand Garret Gang's comment into an answer. The book is A World Without Heroes from the Beyonders series by Brandon Mull. The people who die and are reincarnated are the Amar Kabal. In chapter 18 Jasher tells Jason and Rachel:
Jasher paused, regarding them with icy blue eyes. “‘Amar’ means ‘seed.’ The Amar Kabal are the People of the Seed.” He turned his head and lifted the roll of hair. At the base of his skull was a raised portion of flesh the size of a walnut.
Jason winced. It looked like a huge cyst. “What’s it for?”
“The amar is the vehicle of our immortality. It dislodges at our death, granting new life when planted in the earth.”
“You mean you grow?” Rachel asked in amazement. “Like a plant?”
“Buried in fertile soil, the man grows from the seed within a few months. Less fertile soil requires more time. If my seed dislodged in extremely arid terrain, I might never be reborn."
And there is indeed a ceremony to mark their first death:
Jasher laughed as if the idea were absurd. “No, we are reborn into the prime of adulthood, the age at which we first die. Our First Death is a ceremony held around age twenty."