Just started Kahneman's book recently. I read Ehrenrich's a couple of years back. I can't remember why now, but I was unimpressed. If I recall correctly she mostly attacks a straw man caricature of so-called "positive thinking", focusing on silliness like "The Secret" but spending little time (if any?) on quite a large body of research on "attitude". I do recall thinking she desperately needed a dose of a more positive attitude herself!
I read it recently, and while I agree there's less direct analysis of the body of evidence, her main point is that the 'silliness'
is modern applied positive thinking. The research is irrelevant to the adherents. None of those movements are based on the research anyway, except as cherry-picking exercises to goose up legitimacy.
Where she does address the research, she believes that it's very weak, despite being a 'large body' - she mentions the most referenced experiment (Spiegel - not to be confused with Siegel) was never even completed, and that the next rank of most referenced did not have good inclusion criteria.
I actually circled back to those (I'd read some earlier) and she has a good point... Carver, Lehman, & Antoni for example, and also Schou, Ekeberg, & Ruland's landmark papers all suffer from recruitment depending on participation in group therapies. Unfortunately, group therapies also often reject members who are diagnosed as terminal or are too negative. This biases the survivor / positive attitude correlation toward the positive.
The key question is not whether there's a relationship between positive attitude and survival rate in these conditions, but whether a person can wilfully 'adopt' a positive attitude on demand. The evidence is not only that this is impossible, but that the attempt to force the issue causes precisely the emotional stress that evidence shows is harmful.
The conclusion based on these lines of research is that the movement is more harmful than beneficial, even if it is true that there could be better health among those with more positive life attitude. It is likely a strong genetic predisposition (there is very strong evidence of this in twin studies of personality) and calling people failures for 'incorrect attitude' is likely not only damaging, but probably unethical.