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Do your homework. Read book reviews. Spend time in a bookstore. Figure out where your book would fit on the bookshelves. Find ways to differentiate your idea from the competition, and ask yourself what separates your profile as a new author and your book topic.
Understand that your success will likely hinge on your ability to market your book—and yourself as its author. For more information on how to improve your proposal and platform, check out books such as The Essential Guide to Getting Your Book Published, by Arielle Eckstut and David Henry Sterry, and Create Your Writer Platform, by Chuck Sambuchino.