criticising a book

[defamationquestion]

I am going to try to write this to make sense but obviously leave out details. I have read a book which is aimed at a sort of SN market which I think is out of date, not in line with research, but the authors have marketed it extremely well for years, they are clearly making an absolute fortune out of sales, and they have a lot of good reviews from users

I want to be honest about it online, but I am fearful of retaliation and possible a libel claim, because sales of this book and the associated "events" (which users pay to attend...) are the author's livelihood

At the same time I think it is exploitative and not doing the children affected any good and I would like to raise awareness of this

You’re more than entitled to give your opinion on a book online.

Not a lawyer but I imagine subjective opinion is ok.
So “I don’t agree with” rather than “the author is an exploitative liar”

its fine to post your opinion of the book provided it is truthful. So you can say the text does not tally with recent research (tag it). its not libel if it is honest.

You can give an honest opinion. If you want to say that it is not in line with research, that is a factual claim so you would need to be able to back that up with evidence.