Adding quotations to a nonfiction book

I have finished the first draft of my book. Since nonfiction books, including those for a general audience, often contain (pithy) quotations, I want to add more to my book. I don’t know that it needs them, but since others do this, I think I need to do so too. My question is this: do I need to use quotations only from people I expect my readers to know about, or will any published author with something worthwhile to say on the subject do? What about quutations from books on the same/similar topic?

My book is on praying Scripture. For example, I can probably find a quotation from Timothy Keller’s book on prayer. I don’t know that it will help make my case, but readers probably know who Keller was.

What do you think? Thanks.