An old proverb says, "The devil is in the details." A similar saying, however, is that "God is in the details." One self-published book may be so riddled with fiendish grammatical errors that a reader throws it down in disgust. Yet another self-published book will be a pleasure to read, resulting in a transcendent mind-to-mind communication between author and reader.
Sometimes the difference really does come down to good editing. Once the details are taken care of, the entire book snaps into shape. If you self publish your book, it needs a professional edit because you won't have access to a publisher's copy editor and proofreader. Even the most accomplished authors must go through the editing process when they sign with a publishing house.
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Too much is at stake, financially, for the publisher to send a flawed book out into the market. As a self-publishing author, you face the same economic risks. If your written words don't connect with readers, the book won't sell, and the money you invested in printing will simply become a tax write-off.
You may decide to self-publish for a variety of reasons. Perhaps you've written a book aimed at a niche market, rather than targeting a mass audience. Maybe you tried to find an agent and/or a publisher but were rejected enough times to consider other options. Or you might be inspired by self-publishing success stories about authors who were subsequently picked up by publishing houses and their books became best-sellers.
Most of the books I've edited were subsequently printed by established publishers, but I've also edited a few manuscripts for authors who self-published. In their choice of self-publishing companies, some authors were happy with the experience, while others felt angry and "ripped off" by predatory clauses in the publishing contracts.
It's important to thoroughly research the self-publishing company you choose and to have a lawyer read the contract. As some companies are very laid back and don't ask their clients to do anything except pay for the printing. They say that the author is the publisher, and they are simply the printer.