This book wasn't written for me, because I'm not a pre-teen girl (though there are many adult elements to this, and some foul language, so parents of young readers beware). But I'll go ahead and weigh in anyway. 
 It was a compelling read, in that the degradation of Earth's rotation leads to worldwide calamities. How do societies respond? Unfortunately, this enthralling set up is burdened with the main character and her story. It's just so full of about-to-become-a-young-woman angst. And I couldn't help but see the Slowing (Earth's rotation is slowing) as simply a metaphor for the soon-to-be teenager's feeling of being out of step with the rest of the world. That's what the title is all about anyway: the age of miracles being a reference to middle school, when one's body and mind undergoes all the changes of puberty. But it's a quick read.