I will attempt a no-spoiler review for The Different Girl, and hopefully describe why it was just okay.
A lot of the book revolves around the education of the four girls, with that routine interrupted when a shipwrecked girl arrives at the island (the "Different Girl"). The pace of this learning is very slow, and any thought-provoking insights are easily lost. Did the arrival of the girl change the other four, or were they already changing?
The arrival of the girl (or the storm that washed her ashore) also triggers the second part of the novel, which accelerates the pace towards the ending. This part went well enough.
This leads to another point, the end. No spoilers, but there basically isn't an ending either. Major change, yes, and picking up the pieces in the aftermath, but then it just stops.
Finally, the overall theme without spoilers. Things aren't as they seem from the beginning, with clues about what is really going on. Nothing is stated, however, as the book keeps solidly to one girl's point-of-view, and she never really gets it. So the book ends with the mystery more-or-less resolved, but never stated. Sherlock's summing up would have been out of place, but would have wrapped the story nicely.
There you go - a few nice bits, but slow and no resolution means I can't recommend it to anyone.