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).