Believe it or not...
With nonfiction, you do not have to be a great writer to write a great book.
What you need is a killer concept that meets readers' felt needs and to be able to communicate the basic bulk of it in writing. What you need, more than anything if you are just an OK writer with a great concept, is an outstanding editor to help make your writing cohesive and sparkly, one who knows how to make your power points pop.
Even great writers know this more than anyone: they'd be no where without their editor.
Bottom line: write the book; hire a great editor. Give your message its best chance to powerfully collide with the hearts and minds of your readers.
Now before you get carried away with all the possibilities, let me reinforce the following:
Knowing that you will work with an editor, does not mean you don't carry your load as a writer. Good, excellent, or average writer, serve and honor your message--do it justice--by doing your best to write well. Don't throw it on the page and leave it for the editor to pick up. There's a saying, "You can't polish a turd." I don't actually think that is entirely true when it comes to books based on my experiences. LOL!
If you work with an editor to help you and give feedback from the beginning as you flesh out your concept--the outline and summary--your writing will turn out stronger.
Expect that your initial editing process should be more than just grammar and spelling fixes. The first level of editing after writing should be a good substantive/content edit. It should cause you to think deeply about how your content flows from one point to the next. It should cause you to evaluate how your examples, illustrations, anecdotes, statistics, research, data, and/or personal stories move the reader from one step to the next in their journey through your book. You should expect to do some level of revising after the first level of editing. Then you should expect to be edited again.
That's all for now. I didn't want you to think I was letting off the hook too easily.
This is how editors help you shine. We care about you and your book being great and, more, we care about how your book will make your readers being even greater.