When I use the term “voice” here, I’m talking about the kind of language you use. Regardless of the subject matter, you should express it differently depending on whether you are writing for an academic/technical, business or personal reader.

Here’s what I mean. Let’s say your book is about arthritis and joint pain. 

If your intended readers are medical professionals, you will use technical language to explain how the problem is caused. If you offer solutions to the problem, you will couch them again in medical terms, cite your sources and be cautious in your recommendations.

Perhaps you are a business person writing the book as a promotional tool for joint pain creams, pills or other remedies. You would use less jargon, make the material more reader friendly, and write in a persuasive tone to encourage people to buy your product.

If the book is intended purely as a helpful guide for pain relief for people who suffer from joint pain, you would again use everyday language instead of jargon. You would write instructional material on exercises etc. in a way that is readily understandable.

Same subject, same material, different approach and language for three different types of reader. If you write for one group using the language of another, your readers will never get through your book, so make sure your voice is appropriate for the target reader you are writing for.