To be honest . . .
But if you are going to invest months or years of your life in writing a book and seeking publication or thousands of dollars of your own money to self-publish, you really ought to be honest with yourself about why you want to do that.
There are many reasons individuals give for writing books, and all of those reasons are valid. But the book has to match the reason. When you come to me and say you’ve written a novel, I expect the manuscript you send me to be a novel, not a thinly disguised vendetta against your ex or a memoir about how a lousy surgeon or a hack lawyer did you wrong. If you tell me you’ve written a how-to book, don’t send me a political screed. (And if you tell me you’ve written a political screed, don’t send me a how-to book.)
Because I’ll find out. There will be no secrets that you can keep from a good editor. But lying to yourself and lying to your editor can put you in an awkward position: you’ve committed to publishing something entirely different from what you said it was going to be. And looked at in bright sunlight, when all is said and done it may not be a book that you want to spend time and money marketing, even though you’ve spent time and money writing it and publishing it.
If your real passion is to stand on a soapbox and tell people in the park what a horrible place this world is and how you would make it better if you had the power to do so, then you may have no passion left when I excise your soapbox declamation from your action-adventure novel (because it’s out of place there).
Before you start writing a book, decide why you want to write it.