Based on your review, I purchased 'Not One Of Us' as an e-book from Amazon UK, at the very reasonable price of £1.00 and finished it in 2 sittings.
My verdict? A well-written and thoroughly enjoyable crime/thriller. Unlike some reviewers on Amazon, I thought the structure of the book worked well. The two protagonists were well-drawn. A couple of plot-points stretched credulity a little and I did guess the culprit fairly early. However, this did not detract from my overall enjoyment. As you observed, the - unexpected - epilogue was outstanding. Four and a half stars from me.