A Murderous Procession (Mistress of the Art of Death)

Tess Garritsen calls this one "my favorite book of the year!"

In 1176, King Henry II sends his daughter Joanna to Palermo to marry his cousin, the king of Sicily. Henry chooses Adelia Aguilar to travel with the princess and safeguard her health. But when people in the wedding procession are murdered, Adelia and Rowley must discover the killer's identity, and whether he is stalking the princess or Adelia herself.