After the first few chapters I was about to give up. The internal monologue/narration from our tween heroine Charlotte was just so unrelievedly sour, cute, ironic, self-centered, world weary and phoney that I wasn't sure I could take much more. Along with all of that, the cutesy asides from the author, and the overall forced feeling of the writing was already getting wearing.
But then, something wonderful happens. At page 37 Charlotte's cousin Zee shows up, the plot starts filling in, Charlotte calms down, the author butts out, and the book takes off. Charlotte stops being a whiney know-it-all sadsack. The author stops adding a rim shot to every funny line. The plot stops wandering. Interesting secondary characters appear. Some very clever and subtle deadpan humor seeps into the narrative. The book becomes interesting, funny and entertaining.
The book is sort of Percy Jackson-lite. The Greek god angles are all clearly explained. The humor is broader. The adventure is less complicated. The characters are less developed. The villains are more sketchy. There's less ambiguity about what's going on. The whole work is just more accessible for a younger reader.
So, if you want to try a "Greek god" fantasy and your reader may be a little young for Percy, it seems to me that this would be a really good bet.