The current minimum order required for free shipping on Amazon is $49 for non-Prime customers. The free shipping minimum was originally $25, before Amazon raised it to $35 in 2013, and to $49 on February 22, 2016. Book orders are unaffected by this price hike, and require only $25 for free shipping.
The purpose for these free shipping price changes is likely due both to Amazon's desire to push Prime subscriptions - which offers free two-day shipping plus unlimited video streaming for a $99 a year - and their own shipping expenses - which grew 37 percent to $1.8 billion in 2015.