Girl Unknown by Karen Perry is a simple idea, a girl who turns up (in the first chapter) claiming to be a daughter from an old relationship. The shadowy girl increasingly begins to disrupt the father's lives and eventually the rest of his family. The book is all about relationships, nearly all of the characters have relationships, and few of them have happy ones. Those in relationships sometimes have fault lines, which others can expose.
The problem for me with the book is that the author has had a good idea, but she's constrained by literary conventions to have a climactic ending and the obligatory twist or two. This book never quite makes it from a good first half to a satisfactory conclusion and the ending, and the events leading up to it feel contrived. Like the relationships it details, it would be better of stuck in the middle somewhere and without the need to have a finish. 7/10