Many authors are hesitant to invest in online book advertising due to the cost and uncertainty of its effectiveness. Per W Publishing Group Senior Marketing Manager Katherine Hudencial, Google Ads and Amazon Marketing Services can be great places for authors to test advertising because they have proven to be effective for her authors with a fairly low investment.Hudencial is a big fan of Google Ads due to it's specificity with audience and power of great search terms. She also likes Amazon Marketing Services for authors because you are advertising to a captive audience already looking for books. Amazon also provides a lot of data on ad performance, which can be very beneficial when determining what ad strategies work best.Outside of Amazon Marketing Services, Hudencial advises authors it can be difficult to know which ads convert people into buyers from which retailer and on what day.