Every writer, no matter how skilled or experienced, needs other eyes on their manuscript before it is finalized. The best first step is to find a few trusted people willing to speak truth to power. People who might be your friends or relatives but who, for the good of the book, are not willing to bring the sugarcoating to their comments and are willing to be frank, straightforward and constructive. Soon enough authors will run the gauntlet with an editor assigned by a publisher and the whole process might begin again. Good and really great beta readers, however, can make the difference between a book being purchased for publication and one being rejected. The process assumes that the author has an open mind and a willingness to accept criticism.
A very thorough description of the Beta readers' importance and function.
Though I am a veteran novelist and editor with 50 years' experience, I still find the input of Beta readers an essential phase in the successful development of my personal writing. Simply put, every writer has blind spots. These blind spots may be simple errors of punctuation or larger issues of character development or even plot inconsistencies. A good Beta reader will note and comment on these failures and provide suggestions for improvement. A good Beta reader is an essential tool.
This is important - and reminded me of when I was a producer at Court TV a wise producer told me that when I watch my own "packages" (i.e., teasers for trials or segments) we should view them three times as three different audience members: first, in a supervisory role, looking for errors in fact or mistakes in production, such as editing, continuity and images; second, as the creator, looking for production values and structure; lastly, as the citizen viewer, to see if the video is powerful, thematic, tells the story and tells it well. It was the most valuable lesson I learned as a television producer, and being a beta reader cannot help but make a writer into a better version of themself.