The Sunset Gates — Kathleen Duey

Heart must find her family and a safe haven for the unicorns. A drawing in the book from Dunraven's castle shows the same unicorn design as her foundling blanket! Perhaps the book tells more about her family -- but she cannot read it. Searching for a teacher, she finds a blacksmith whose family forge was made by unicorn magic. His old records show that his grandfather made giant iron gates using the same design. If Heart can find those gates perhaps she will find her family -- but will Dunraven's men find her first?