Bottom line: this book follows the Lemony Snicket three fold path - engaging characters, interesting premise, precious annoying narrator. The good news is it does so without the vaguely sour and condescending Snicket aftertaste I think you can sometimes get from Lemony.
The book's heroes are particularly strong. There are two siblings who complement each other, and are loyal to and supportive of each other. Like minds with distinct talents, they make a great fictional team. A third kid is added to season the mix, and he adds charm and street smarts. So you start with good basic ingredients, (and none of that sibling infighting that can get very old very fast if not handled just right). Secondary characters don't add much to the plot as such, but again season the pot nicely. Aunt and Uncle are sort of a hoot as vague adult figures. Neighbors and tangential characters come and go, and mostly serve to keep a little variety going. The villains aren't terribly well developed, but they aren't really the point of the tale, and just sort of serve as placemarkers, since you have to have a villain to make the tale work.
The premise, (MILD SPOILER ALERT), that you can bring book characters to life in our world through an enchanted alchemical circle, has of course been done before. From the "Magic Treehouse" kids who travel through time and space, (for the second grade crowd), through "The Time Warp Trio", who travel into the past through a magic book, (3rd and 4rth gradish), to any number of books for older middle-graders, this is an honored framework. The angle here, that you can animate fictional characters from classical works, (as opposed to historical "real" characters), adds a nice twist to the convention and leaves lots of rooms for sequels. And, the choice of Quasimodo the Hunchback is especially interesting, since he's probably not the mainstream first choice.
The "but" here is the narrator, who will fall for each reader somewhere along the "vaguely annoying to makes it unreadable" scale. The narrator is drawn as an actual character in the book who has been asked by the children to write about their adventures. That makes him a lot more present in the story than just some unseen, unknown narrator. So far, so good - nobody hates Dr. Watson from Sherlock Holmes. But, this narrator has a running grievance about being a janitor at a college, which is just distracting and beside the point. The narrator also adds parenthetical definitions for words in the text. This was sort of interesting and sort of helpful at the start, but it broke up the narrative, got too cute and then got affirmatively annoying pretty quickly. I think the problem is that it's show-offy without advancing or enhancing the story. (The narrator also does this meta-fictional thing where he explains why the book is being written a certain way, and while this can be interesting it also robs the story of any momentum by always making it clear that this is just a made-up fiction.)
The upshot is that with engaging characters and a sort of thinking-man's storyline, this book could be a hit with your reader. It doesn't seem like a sure thing, and it's vaguely old-fashioned, (no hip references or gossip-girl type vibe), but it has a certain idiosyncratic charm that could engage your reader.
Please note that I received a free electronic copy of this book from the publisher in exchange for a frank review.