It's more of a misconception or disempowering perspective than it is a myth, but it's such a common sentiment I hear on a regular basis. I thought I'd address it today. 
The myth is: “No one is going to read the book anyway.” 
This insinuates that what you put in your book doesn't really matter.
It's is dangerous for two reasons. 
The most important reason is that rich and successful people read. If you're building any credibility by "just" authoring a book, all of that credibility fast dwindles if the contents suck. The second dangerous element is the pernicious mental slothfulness it can promote. It's like suggesting to yourself that no one cares about what you have to say. That is not the type of messaging you want to tell your subconscious.