I've really enjoyed the Shetland series thus far, but it all goes wrong when Jimmy Perez returns to his native Fair Isle, with his fiancee Fran, to visit his parents. Then someone is found stabbed and as the weather is too bad to bring in more people Perez must investigate on his own.
One of the most pronounced and unusual things about the preceding three books was the way in which Jimmy investigated crimes through the network of island associations and rumour. This is abandoned in this book as the only defined island characters are Jimmy's parents, the rest of the story deals with outsiders at the bird sanctuary. The claustrophobia that Cleeves may have been trying for as a trade off didn't really work for me.
The ending is a bit of an anticlimax, with a couple of gratuitous plot twists at the end that left the next 20 pages feeling a bit superfluous. Again, I can see what she might have been aiming for, but it didn't really come off. And its hard to imagine the charcoal document would have any legal standing or be acceptable.
Overall, I wish I'd stopped at book number three. I wish the author had as well, as its fairly easy to predict which way the story will go after this - if it returns at all and I'm not sure I trust her any longer to deliver the goods.