See more of my reviews sooner on The YA Kitten! My copy was an ARC provided by the publisher via NetGalley.3.5 stars!Admittedly, I didn't know this was a middle-grade novel until I started reading it. That didn't stop me once I got into the novel; if a book sounds like a great book, the genre doesn't matter to me. I know I'm not the only person to make that mistake with this novel and hope they'll also read it anyway. Tubb plays on my love of the twelve zodiac signs perfectly and weaves a fantastic tale.The 13th Sign is a page-turner from beginning to end, with well-written action scenes throughout and some ingenious plotting making the novel that much more interesting. Jalen doesn't always seem like a real girl and I often forgot her name (whoops), but her problems were real enough and I felt bad for her. Her friends, unfortunately, didn't get much depth either, but this trio of kids struggling against the twelve Zodiac Keepers is easy to root for once they get going.Most of the novel's problems were relatively small, like how supposedly everyone has had a change in horoscope/personality with the release of Ophiuchus. Not true. Anyone born on April 19, May 14-May 20, June 20, July 21 or 22, August 10-August 22, September 16-September 22, December 18-December 21, January 19, February 16-February 18, or March 12-March 20 retains their original horoscope when Ophiuchus is added to the zodiac.With 7 billion people in the world, that averages out to 19,178,082 people sharing a birthday (though I do realize this is just an average and not how many people actually share a birthday). With 48 days on which people can be born without their horoscopes changing due to Ophiuchus, that means 920,547,936 people are unchanged. That's only about thirteen percent of the population, but it's still enough people that such a statement becomes false and it spends the entire novel bugging me.There were also some references to the orange inside of Jalen flaring up. I know she means her anger, but referring to it as the orange is very... odd, and out of sync with the otherwise straightforward story. As far as I know, pink has no smell and Jalen does not have synesthesia, so where did someone's breath smelling pink come from? Saying their breath smelled of bubblegum would have been simpler. And if there is a kind of bubblegum that makes one's breaths turn pink, I want some. Now. I want it to be pink when I breathe out!Still, I enjoyed The 13th Sign overall and I'd be happy to reread it in the future to get a better sense of its subtleties.