While any single review may not be helpful to an author, I think collectively they can have value. Especially in the case of a non-fiction book, if I have 10 critical reviews and there is a common theme to the criticism, such as ‘there was too much repetition in the second-half or… I couldn’t follow the explanation of how to… or The author sounds really self-congratulatory or… there are errors in the sample programs. Well, then I think the author should see those criticisms. Even with fiction, there can be a common thread to the criticism that is worth considering in your next work. Just my 2 cents…