First of all, it’s GREAT that you’re planning to use beta readers. Getting input on your beta draft is one of the best ways of ensuring that the final book will really be useful for its intended readers. For that reason, it’s important that at least one of your beta readers qualifies as a target reader! We suggest that you aim for 4-6 beta readers in total: as well as target readers, you might want to choose experts in closely related fields, or professional connections who can add a helpful perspective. It goes without saying that they need to be capable of constructive criticism and also that they need to be reliable – the most insightful reader is no use to you if they don’t send you their feedback.
Don’t just hand over the manuscript with a vague request for feedback. Think carefully about what each of your beta readers can most usefully offer: for a target reader, for example, you might want to ask whether they understand all the terminology, whether the argument is pitched at the right level, or whether there are specific use cases or issues that aren’t covered. For a professional peer you might want to ask whether you’ve incorporated the latest thinking and research, or whether there are any gaps in your coverage. But always give your beta readers the freedom to give feedback that you DON’T know you need too, eg finish with a phrase like ’These are just suggested questions to keep in mind as you read: I’d be grateful for any other thoughts or suggestions that occur to you in addition.’
You may get conflicting advice from different readers: it’s your responsibility as the author to consider all the various perspectives and decide whether and how you take those suggestions on board.
And don’t forget to invite your beta readers to provide an endorsement for the book (if they liked it…), and to thank them in your acknowledgements too!