There seems to be tendency among some self-published authors (and maybe a few legacy published ones, too) to believe that the world of books is a zero sum game. This mindset is potentially fatal for a writer. Don't fall into the trap of thinking of your book as competing (either directly or indirectly) with other writers' books, even in the same genre. Readers simply don't approach books the way they would, say, buying a car. Just because someone buys Book A instead of your book (both, say, historical fiction), doesn't mean if Book A had never been published that reader would have bought your book instead, and vice versa. Nor does it mean that if Book A can garner enough crappy reviews, your book will outsell it, which leads some authors to either ghost bad reviews for the "competition" or solicit their own loyal readers to do it. The only time books compete head to head is in an award competition where you know the reader/judge is going to be reading BOTH books and picking a winner. So don't view your fellow writers as competitors. After all, readers can buy their books and yours.
Lissa Bryan did an excellent post last week in her blog on this same topic.