I am not perfect: never have been and I never will be. The fact that I couldn't figure out how to avoid ending that first sentence with a preposition likely proves my point. Sure, I could likely spend five to ten minutes rewriting the sentence to fix it; however, I like the way it reads and I believe that it fully captures my point in a succinct way. The question is, will that first sentence feel like a bur in the reader's shoe (i.e. sure they can keep reading along, but is that phrase an annoying distraction)? That's where beta readers come in handy.
Just about everyone has a good grasp of grammar and punctuation to the point where they can catch a quarter to half of the mistakes made in a first draft. Likewise, the average reader knows dialogue that works, and doesn't, when they see it. Most readers pick up on continuity issues and can spot plot holes like the old gumshoe detectives from the 1910's. Readers are good at spotting things that we writers miss. Sometimes this is wonderfully beneficial, like when they find a deeper significance or theme in a story arc or character development. Other times, they catch our spelling errors or gaping flaws in the plot. Catching these issues before publication can mean the difference between the success or failure of your book. This is especially true for self-publishing (or "indie") writers. While the initial stigma of self-publishing is falling by the wayside, an error ridden book comes across as unpolished, unprofessional, and ultimately unmarketable. Authors already working with a publisher won't encounter this issue, as their publisher will use a copy editor (and foot the bill for the proofing).
Beta readers can help you find the flaws and show you where you may need to polish your writing. This may mean tightening up a dialogue scene, or cutting out exposition that doesn't drive the story forward, or adding elements that strengthen some of the plot hooks. One of the first critiques I got from my beta readers was that the book's original title had to go. As it was a working title at the time, I happily chucked it aside. Unfortunately, the second title didn't work either. Eventually, "Of Murder and Monsters" struck me like a rubber mallet to the back of the head. More importantly, the beta readers fell in love with it as well. With all that being said, unless your beta readers are English majors/teachers, you'll still want to consider having your manuscript professionally copy edited or proof read.
Where do you find beta readers and who are mine? First of all, my wife is one of my beta readers. Aside from being able to give me helpful constructive feedback, she's not a fan of the genres in which I write. This is helpful because if your reader is not a fan, yet still likes the material, it's a decent indicator that you may be on the right track. Another beta reader that I've used is a fellow author who writes in similar genres. Having another writer as a beta reader is also helpful because they bring a different perspective to the equation. A fellow author as a beta reader will likely go the extra mile and offer suggestions to help you fix the flaws and refine the better aspects of your book. And that's what writers need, more than anything else, from their beta readers: feed back. While anyone can be a beta reader, you'll want to use people who you trust to: actually read your manuscript in a timely manner; not steal your work; not share your work without permission; and provide you with meaningful, constructive feedback. The best beta readers are the ones who'll write down their feedback and provide you with a short summary that covers their thoughts on: the title; main characters; character development; dialogue; plot development; and continuity issues.
Thank you to all of the folks who were kind enough to be my beta readers for "Of Murder and Monsters." I especially want to thank my wife for being brutally honest about what worked and didn't, as well as for her unwavering patience and support.