I enjoyed this read very much. There are all the usual elements of the whodunit murder thriller, with a good range of twists and blind alleys. I particularly like the main character, the early retired academic trying something a bit different. He seems to me to be a very plausible stoical, rather than heroic, figure that I had a natural affinity with. The heavy hand of the law, the presumptions of guilt or innocence and the tensions around the distribution of family money are the bread and butter of the crime genre. These elements and others are very well employed with varying degrees of originality.
There is enough in the plot for one to be able to get shadows of the ending and yet still be surprised. Agatha Christie, eat your heart out. Here is a modern writer with a similar flare for building plot that encourages the reading into stereotypical thinking and assumptions, only to then turn many elements on there heads. I will be intrigued to see whether Mackey can maintain the art of this genre so well in future books.
Mackey writes with a light touch, conveying the information we need with enough rich background to interest without drawing us so far away that we lose touch with the important plot details. He has also left enough ground raked but not so deeply dug over that it would do much to limit the direction of a sequel or even a series. Rubia, in particular, is a character with potentially miles of interesting story that could be exploited.