A beta reader is somebody who volunteers to read work at an early stage, when the ride is going to be rocky. Like those daredevil types who test new airplanes, a beta reader survives on curiosity and faith.
Some people have the interest and generosity of heart to be beta readers, but are otherwise not experienced enough to be of much help. And a beta reader has to have some knowledge of your genre to be really helpful.
If you’re writing a book about a place and/or time that is not within the sphere of your personal experience, a beta reader who is familiar with those things is really necessary. You may manage with a huge amount of research, but you will miss things that a good beta reader will pick up. Even if you’ve already sold the novel you’re working on and have a great editor at your publishing house, it’s unlikely that your editor will know enough about (say) New Orleans in 1814 or the things Baptist church ladies in the Deep South are most likely to argue about.
Writing a second book set in the Deep South, I am a little more comfortable than I was last time — in some ways. But then again, Tied to the Tracks was centered around a small liberal arts college, and that is an environment I know very well. In some ways Pajama Jones is much more of a stretch. So beta readers? Absolutely necessary.
My Pajama Jones beta readers are friends, a married couple who live in the south. Between the two of them they can identify pretty much every misstep, but better still: they send me links to local newspaper stories and classified ads that provide excellent and genuine detail. And they don’t laugh at my Yankee mistakes. They are firm, but kind. No, I may not have these men playing touch football. Absolutely not, not in that town at that time. No, that is not how you address a preacher from that church. And so on.
Thus I am very aware of the debt I owe to my beta readers. I don’t know what I’d do without them.