It did pretty well considering it had basically every trope I hated about YA books. It was a light, fast read with an original plot, but the world-building was quite weak and it honestly eludes me how this became an international bestseller. First of all, Gwen was extremely shallow and underdeveloped and that's a bad sign because it means that the other characters will be even more shallow. And that was true. But first of all, Gwen has basically no development and makes some strange choices. When I posted my first status for this book, a friend told me she would get better, but in my book she didn't. 
 I kept trying to fall in love with [Mr. Whitman the history teacher] (if only because the boys in our class were all somehow totally childish), but it was no good. 
 So why do you need to be in love with somebody? 
 She screams 'special' from the beginning.I used to long for red hair, but Lesley had convinced me that my black hair was a striking contrast to my blue eyes and fair skin. Lesley also managed to persuade me that the little crescent-shaped birthmark on my temple - the one Aunt Glenda always called my 'funny little banana' - was intriguingly mysterious and chic. I also didn't get how this book was funny. Okay, so I seemed to be in a period when letters were still delivered by carrier pigeons. Or maybe snail mail. Using actual snails. 
 "The password?" 
 Quark edit biscuitus. or something along those lines. And some things I found funny that weren't meant to be funny. Trust no one. 
 Not even your own feelings 
 And now our wonderful supporting cast. Having recently watched a youtube video of the Hamilton cast celebrating "A Chorus Line" and talking about how crucial the ensemble is in a show, it makes me think that the supporting characters in a novel are similar and if they're all made out of cardboard, that does a number to my rating of the book. Most of the characters can be described in a couple quotes or sentences. 
 Lesley is a really archetypally shallow teenage girl who talks about movies and I can't understand how she and Gwen are friends or anything. Like half the best friend characters, she's just annoying. 
 "Wow, cool," whispered Lesley. "Is she going to turn all transparent now?" 
 "Ears can tell you a lot about the person, especially the earlobes." 
 "Investigating mysteries must be be in my blood.... Maybe I'll study history too and specialize in old myths and ancient writing. And I'll be like Tom Hanks in the da Vinci Code.... and I'll hire a really hot guy to be my assistant." 
 Charlotte is a know-it-all 
 and a snob 
 and a know-it-all 
 and a snob 
 and a know-it-all 
 (view spoiler)[and a snob 
 and a know-it-all 
 and a snob 
 and a know-it-all 
 and a snob 
 and a know-it-all (hide spoiler)] 
 "Let's hope it's not in the middle of the Great Fire of 1664." 
 "The Great Fire of London was in 1666," said Charlotte. "That's easy to remember." Gwen's siblings: I think they should be about half their respective ages. 
 Gwen's inexperience is an excuse for Gideon the hot idiot to rescue her. 
 Madame Rossini: First of all Rossini is ITALIAN!!!!!!!!!!!!! 
 "Allo... You must be Gwyneth.... we can't have you traveling in time in zat dreadful school uniform." 
 "Men, always in such a 'urry! But with fashion you cannot 'urry." 
 And then, of course, there's a bad romance too. Initially, of course, Gwen hates Gideon because he's an idiot. But he's a hot idiot, so he changes and they're like sort of attracted to each other (view spoiler)[ until the end of the book, when he reveals that he suddenly somehow loves her. (hide spoiler)] 
 I have book two, so I might as well pick it up soon but I sure hope there's more world building.