I've realised that a lot of free kindle books are simply awful: either the plot is terrible, the characterisation is bad, or (possibly worse) the proofreading/editing and formatting are terrible and/or non-existent.
Not the case with this one, I'm pleased to report. It appears to have been well proofread, it's well written, and I didn't notice any formatting problems that so often crop up. I think there were a couple of minor mistakes, but nothing that stood out.
As to the content, for the most part it was informative and interesting. I certainly learned a lot about lie detectors, how they work, who they work on. The author has some bias that he brings to the book (of course!), but it's clear and he makes no effort to disguise it. There is a little proselytising, but he appears to feel strongly about the subject.
There are a couple of occasions when I think he gets carried away describing specific cases - something that is not his strong point. The Francke murder is a good example here, with a whole chapter devoted to the case without mention of a lie detector, yet with a number of personal opinions thrown into the mix. I'd suggest that chapter was the weak point of the book, and although he does the same thing in a couple of other places, the other instances are briefer.
All in all, a fascinating read though. I would give it 3.5/5, but since it was free I'll round it up to 4!