A beta reader is basically someone who reads your unpublished book for you and tells you what they think. Their role is to be critical, and therefore family members and good friends aren’t always the best choice for the role.
Other writers are a good choice – mainly because they can do the job as a quid pro quo. A job done well takes a lot of time. The beta reader has to read the book. The book may not necessarily be their genre of choice. Then they have to give feedback in a useful and constructive way.
Delivering useful, constructive feedback is a skill in itself. I’ve done it a few times and it will make you a better reader and writer.
Apart from other writers, you also need people who are your target audience. If you’ve written young adult fiction, you need teenage readers and if you’ve written crime fiction, you need fans of Mark Billingham, Christopher Brookmyre, Denise Mina et al.
On a personal level, my beta readers have told me about my writing mistakes. I’m repetitive, my sentences are too long and I use far too many parentheses. The beta reader I used who has an editing background pointed out I’m stingy with commas and I’m too liberal with passive voice use.
Lesson number 1: if you want beta readers, be prepared to do the job yourself.
Lesson number 2: prepare for the feedback to hurt.
Lesson number 3: ask as many people as possible.
Lesson number 4: prepare to be disabused of your brilliance. I have borrowed this neat turn of phrase from an acquaintance of mine, Eric. J Smith, who used the wording to describe what happened when he used beta readers.
Lesson number 5: know you will need to re-write your book.
Lesson number 6: be grateful. Thanks to two of my beta readers, I’m paying far more attention to my word choice and my sentence construction. This can only mean my writing will improve.