You only have one chance to be a debut author. Debuts are special in the world of publishing. While the experience of having gone through the publishing process can make you wiser–and therefore theoretically better at it–there are reasons to try to get it right the first time.
In publishing, your “track”–your past sales record–is everything. When I worked at HarperCollins, we had weekly meetings where we picked apart the sales history of every author on that week’s New York Times bestseller list. It was normal to hear things like, “Debbie Macomber’s track is up 10% from her previous book last year.” We watched sales as if they were price fluctuations in blue chip stocks. These fluctuations represented investments gone sour, opportunities to poach a best-selling author who might not be pleased with their current house, and changing trends in consumer tastes. In an industry where there’s very little real science to determine what books will succeed, the information we did have was dissected and rehashed constantly.