This is a well written story, the pace is good, and the characters are believable and fleshed out.
I'm not sure if I'd choose the bouncing back and forward style between the current story and the Gabi back story, but it works here quite well.
What stops this getting a 5 star is the political stance the author seems to have taken, I found at several points in the story the decidedly anti European bias a bit much.
The views of Christianity / Islam were more balanced and believable however.
The afterword explains it quite well, so is well worth reading, but I do not agree with many of his conclusions, though they are well thought out.
There are a few nasty bits in the story, I think that they could have been described more briefly without losing any plot, so be warned it isn't for the faint hearted, though if you skip a few paragraphs / a page or so you still get the idea of what happens at these points.