Back in 2013, Rob Fitzpatrick wrote a short book called The Mom Test to teach entrepreneurs to ask for better customer feedback. In its first month, it earned a paltry $535. Six years later, thanks to steady word of mouth, it passed $10k in monthly royalties and has continued to grow from there.

It’s now taught at universities like MIT, UCL, and Harvard; recommended by startup accelerators like Y Combinator and Seedcamp; and used by teams at companies like SkyScanner and Shopify. It has hit #1 in most of its Amazon categories and has been translated by enthusiastic readers into nearly ten languages. All of this happened while he was largely ignoring the book and doing approximately zero active marketing. To date, it has put more than $550k in his pocket, all via reader recommendations.