Online reviews vs Evidence of expertise

Let’s say you are looking for a method book on playing a musical instrument, such as the Guitar. There may be many factors in deciding (such as price, front cover, contents and length) but I would like to focus on comparison between the following two factors.

A) Online customer reviews on retailer websites such as Amazon etc.

and

B) Actual evidence of the author’s skill such as videos of them playing their instrument (including to an advanced level).

Questionnaire results are publicly available

If both were available for the same book, how would they matter in comparison?

If you were a beginner and looking for a book for a beginner, would the author showing evidence of their skill above beginner level, contribute to your confidence that it would be a good book?

Between two different books, which would be more attractive to you?

If a book had a poor review or reviews, but the author had evidence of their own skill, would this affect how valid you perceive the review(s)?

Between two different books, which would be more attractive to you?

Do you believe online customer reviews posted over the internet in general are by actual genuine customers giving you their honest views?