Literary agent Rachelle Gardner says the biggest mistake authors make is querying too soon. Authors send the first or second draft of a book, when in fact they need to spend years learning their craft. Read her comments here.
The same thing happens when self-published authors rush to market with their books. Maybe they hire an editor. But they rarely go through the process of studying and planning.
I encourage writers to prepare a formal proposal, even if they don’t plan to publish their books with an outside publisher. Working through the plan will force you to consider your marketing, competition and selling points. If you write self-help non-fiction, write out your sales page first.