Buying books has more effect on the English, maths and science test results of 11-year-olds than cash spent on technology or teachers, a study of more than 6,000 schools reveals.
If schools spent pound;100 per pupil on books, the average test scores for key stage 2 pupils would rise by 1.5 per cent per child.
The findings are based on an analysis of data collected by inspection agency Ofsted from almost a third of primary schools in England, and a survey of 540 headteachers.
Steve Hurd, from the Open University, who led the research, said he had expected initially that the amount spent by schools on books would have no effect on test results, once social class was taken into account.
But his team, which included Malcolm Dixon, of Liverpool John Moores university and Joanna Oldham, of Liverpool Hope university, concluded that the average test score across English, maths and science would rise from 27.5 to 27.9 if schools spent pound;100 per pupil on books.
The equivalent spending on technology would have half the effect.
"It is surprising that books matter," said Mr Hurd. "It may seem a somewhat small effect but it is significant."
But two-thirds of primary schools said they did not have enough money to spend on books after staffing costs - and more than half of heads said information and communications technology was a higher priority.
Mick Brookes, general secretary of the National Association of Head Teachers, said most of the money for school budgets was earmarked for staffing.
"This clearly prevents schools from pushing some funding into areas where they think it should go, on books and libraries," he said.