[WARNING: Unlike some people, I generally try not to put spoilers in my reviews. This will be an exception. Then again, if you still want to read the book after reading this post, you deserve what's coming to you. Also: long and screed-y.]
I read Hannah's first Hercule Poirot pastiche when it came out and thought it was pretty bad. The plot was all over the place, she had a tin ear for dialogue and names, one character showed strong signs of being a Mary Sue, and the narrator character was afflicted with pointless and ultimately trivial psychological hand-wringing.
I skipped the second book. But then I spotted the third in the library, and it opened surprisingly well, and I thought, what the hell. (What can I say? I'm not bad, just weak and easily led.) The good news is that the aforementioned problems have been minimized. The bad news is that they've been replaced by new, much worse problems.
Look: if you're going to pastiche Agatha freakin' Christie, the third-best-selling author of all time (behind the Bible and Shakespeare), there's one thing you've got to have. You've got to have a puzzle. This is the one fundamental overpowering thing that Dame Agatha did better than anyone else before, during, or since. She'd set up a puzzle, give you all the clues, and then pull out a solution that (a) you didn't see coming, and (b) seemed totally, logically, inevitable. That "aha!" moment--or, more specifically, that "I can't believe I didn't see that!" moment--is why people read Christie in the first place.
Sophie Hannah's moment is not an "aha!" moment. It's a "chuwhuuuuh?" moment. Actually, it's a series of "chuwhuuuuh?" moments--an elaborate, rickety structure of improbable psychological hand-waving combined with utterly nonsensical internal logic. There are too many ridiculous bits for me to describe them all. A few particularly egregious examples should give the flavor.
The central clue that makes no sense whatsoever.Throughout the book, an enormous fuss is made about identifying the typewriter that produced several letters. Let us suppose that you are the villain, and do not wish this typewriter found. Do you:
If the answer is #1, for God's sake, WHY?
The absolutely ridiculous fundamental premise.
Your cunning plan is to accuse your sister of murder, so that she will be hanged and you will get the money. The death in question was, and was assumed to be, natural. You write letters vaguely asserting that there has been Foul Play. Which of these things do you do?
I could go on . . . and on . . . but why bother? Sophie Hannah might well be a good writer for a different sort of book--there are flashes of that--but as constructor of puzzles she's absolutely hopeless.