I found the ending exactly what it should have been. Adam/Rob is a very damaged character, and as you say it makes sense. He doesn't know what happened, all his flashbacks only serve to confuse him further, so why should we be less confused. I think it's sad these days that readers want every i dotted and t crossed. The present day murder was solved (and yes, I fingered R from the start, couldn't see why nobody else did), but the ongoing case, seen through the rather surreal narration of Ryan, was rivetting. All the main characters came over strongly, all damaged in their own way, and Ryan's inability to understand Cassie's suffering was very well drawn, even when you wanted to shake him!
I see that Tana French is nominated for first novel by an American author, but for me she does not write as an American (and indeed she seems to have spent most of her life outside America). I don't know Ireland well, but I found the background most convincing.
I loved this book, and as I've said elsewhere, I need to read it again. I picked up on a lot of Ryan's 'hidden' points, but not all.
I see on Amazon that her new book is about Cassie, which seems right - can't see another book devoted to Ryan, poor soul!
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Now reading: Fade To Black - Francis Knight