Eff is a twin and the thirteenth child in her family. Her twin brother, Lan, is supposed to have great good luck and magical power because he is the seventh son and their father is also a seventh son, making Lan a double-seventh son, a powerful combination in magical numerology. But the thirteenth child is supposed to be a curse and full of bad luck and evil tendencies. Eff is discriminated against by the magical community, but her family and her twin brother stick up for her. When the family moves to the very edge of the frontier in the American West, Lan and Eff will face challenges and their magic will be tested to the utmost.

I really loved Eff's character development! The story follows her from the age of 5 until she is 16 years old, so we get to see as she matures and grows through the years. Being the thirteenth child hangs like a shadow over her life, but she works hard to prove that she is a good person. Still she fears that might bring bad luck to her family. She has to struggle past those fears and embrace her magic.

The magic system and world-building of the western frontier is really interesting! There are dragons in the mountains, and wooly mammoth creatures and weird little magical creatures all over the frontier. The settlements have trouble keeping out the dangerous animals, so they put up protective spells all around their little settlement towns, but some people are prejudiced against magic and they insist that the settlements would be better off with no magic at all.

I'm hoping that I can read the rest of this trilogy, but the other two books are really difficult to find.