Author Jeanine Cummins’ planned book talk at the Petaluma Copperfield’s Bookstore evaporated with the rest of her scheduled appearances last week when the publisher of her novel “American Dirt” canceled her book tour out of concern for her safety. Flatiron Books president Bob Miller said they shut down the tour after threats of violence against Cummins and certain booksellers. The backlash contrasts with the attention her novel received as Oprah Winfrey’s latest book club choice. On the…