The Dangerous Life of Agnes Pyle - Sentry Saga, #1

It's the life she never wanted, but she must live it. or die. For longer than human-kind can remember, Sentries guarded the barriers between the mortal and spirit realms. Their sole mission was to ensure that the two worlds remain separated, guarded from any and all threats. Though mortals themselves, they were graced with abilities no mortal or spirit could ever hope to possess. Those spirits, demons, or even mortals that threaten to pass the barrier would find themselves prey in the endless fight between good and evil.
But now it is the Sentries who are being hunted. Having lost her parents, Agnes Pyle must face her destiny to join the ranks of the guardians between the two worlds. Though she is too young yet to do so, she is forced into her apprenticeship with Brandt, an older, powerful Sentry already teaching another. Meanwhile, a former apprentice of Agnes' parents, Safiya, must flee her home or die, while hoping to be able to make it back to Agnes to help protect the young girl.
Demons hunt down Agnes as signs of her power show sooner than any could have guessed. She must learn to fight or she might not survive.