I love the idea behind this book, the mysterious thirteenth zodiac sign and how it ties in with everyone's personalities and actions. This part is explained really well and I found myself getting into the story. However once Jalen starts on her journey to challenge the other twelve signs of the zodiac, things took a turn. The story just isn't believable - not in the sense that it's a fantasy novel, but because the decisions Jalen makes and her actions don't make sense. One of the things that really frustrated me is that I, with my limited knowledge of the zodiac and mythology, was able to correctly guess who that new character was and what they were going to do way before Jalen did. It's unbelievable, especially for a girl who supposedly grew up on this kind of knowledge. The 13th sign isn't aimed at me, but younger readers (elementary school) will probably find in it a thrilling adventure and great friendships.