So, here's the review:
Rarity and The Curious Case of Charity
Technically, I'm very wary of licensed novels. Either they're 70-page commercials or they're a lazy retelling of a certain story previously featured. However, since G4 proved to be better than I thought it would be, and as I thought the comics were pretty entertaining, I decided to give its chapter books a shot. Rarity's book sounded interesting to me, plus, Rarity is best pony (geez. i don't know how many of you share that opinion).
Without giving any huge spoilers, the plot of Rarity and The Curious Case of Charity is simple. Hired by an fashion-oriented organization known as H.O.O.F (House of Outrageous and Opulent Fashion), Rarity is scheduled to assist in mentoring a fashion student who calls herself 'Charity'. As time goes by, however, Rarity begins to notice that there's something slightly...off about Charity, as it seems like she's taking the instructions 'just do as I do' a bit too far.
As a book marketed to children, this plot line, while generic, is actually quite interesting when you think about it. Children tend to have a bit of talent in mimicry; I in particular tried to imitate a popular girl a long time back ago, but didn't go so well. I suck at making friends, much less jumping myself out at random people, and I didn't have her slapstick sense of humor at all. Best of luck to that person, but I'm not her.
The characters are alright; Rarity is mostly in character. It's not hard to imagine her doing the things she does here. One thing I did notice is that she occasionally notes what's in fashion, which is, while not completely out of character, generally not something I'd expect her to do past Season 1. The other canon ponies are in character too; Sweetie Belle in particular has an adorable moment, but I mm not going to spoil that for you. Oh, and a certain mailmare makes an appearance!
Charity is interesting. As I said, she could serve as an archetype for children in a similar situation with their environment. On the plus side, while she's also into fashion, she also expresses excitement at Applejack and Rainbow Dash's less delicate activities, such as apple bucking and canon testing. Honestly, there's nothing wrong with being specifically feminine or a tomboy, but seeing a character like this is quite refreshing. Some may be a bit ticked off by her, but I generally like her.
Overall, the characters are alright. Nothing outstanding, but except for a few incidents everything's generally in place.
Like most basic chapter books, the flow, while easy to read, is unfortunately not very unique in terms of style, and occasionally get kind of tedious. At the very least, it isn't extremely simple, neither is it something like 'Rarity tucked each dress in carefully, making sure each one was perfect, fluffy and pretty!' or 'Sweetie Belle hoped up and down in sugary glee'. Blech. The prose and plot are overall not bad, but could be better.
As for consistency with canon, like I said, the ponies aren't very OOC and no laws of magic are broken or such. One thing that's a bit of a nitpick is that the Mane Six gather at the Castle of the Two Sisters just to talk once, and as far as I'm aware, this isn't something they would do when the matter is relatively trivial. Perhaps at Golden Oaks (this story takes place mid S4-*sniff*), but not there, especially not as soon after the Castlemania incident.
A few additional details-as I said, there's some fanservice (not that kind) in the book, with call backs and a few cameos. It does seem like G.M Berrow actually has watched a fair few episodes of the show, which is good. Kids who don't watch the show may be slightly confused at some mentions (Discord is referenced once) but they'll generally be entertained by this, most likely.
Overall, the book is alright. It's not the best Expanded Universe installment with Rarity as a focused (The Nightmare Forces arc from IDW comes to mind) but it is an enjoyable read if you're a fan of Rarity. Do try to read it if you can, though you probably shouldn't spend you three dollars on it at Amazon.