If all schools could be persuaded to use textbooks in the top 20% of quality as measured by educational outcomes, the average student would perform at the level of someone currently 3.6 percentiles better than they do now.
It is much easier to quickly transform the textbooks students use than a significant share of the teachers who use them. And, while the gains may seem modest, they are impressive in an area where the dominant predictor of outcomes are student socio-economics, while teaching process impacts (e.g. percentage of certified teachers, teacher pay, student-teacher ratios) are rarely measurable except at the very top and bottom extremes. Also, generally, better textbooks are not more expensive than inferior textbooks.