There are two basic ways to sell a book. Either you market the book or you market yourself. You may have heard an "expert" talk about the importance of becoming an expert in whatever you've written about? Of course this applies to non-fiction mainly with the idea being that when people think about your subject they think of you. The upside is that you can build a very solid platform in a relatively short amount of time. The downside is that you can get locked into that particular subject.
My book, The Troubles, deals with a lot of heavy duty stuff, but I don't see myself going back to school to obtain a psychology degree just so I can appear to know what I'm talking about. I'm confident enough that my story speaks for itself, and I don't envision writing another of the same type.
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