The Queen of the Tearling / Erika Johansen.

On her nineteenth birthday, Princess Kelsea Raleigh Glynn, raised in exile, sets out on a perilous journey back to the castle of her birth to ascend her rightful throne. Plain and serious, a girl who loves books and learning, Kelsea bears little resemblance to her mother, the vain and frivolous Queen Elyssa. But though she may be inexperienced and sheltered, Kelsea is not defenseless: Around her neck hangs the Tearling sapphire, a jewel of immense magical power; and accompanying her is the Queen's Guard, a cadre of brave knights led by the enigmatic and dedicated Lazarus. Kelsea will need them all to survive a cabal of enemies who will use every weapon - from crimson-caped assassins to the darkest blood magic - to prevent her from wearing the crown.