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.
It shows that the average test scores for key stage 2 pupils in schools which spent pound;100 per pupil on books rose by 1.5 per cent.
The findings are based on an analysis of data collected by Ofsted from almost a third of primaries and a survey of 540 headteachers.
Steve Hurd, from the Open University, who led the research, said he had expected the amount spent on books would have no effect on test results, once social class was taken into account.
But the research team, including 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 in schools which spent pound;100 per pupil on books. The equivalent spending on technology would have a smaller effect.
"It is surprising that books matter," said Mr Hurd. "It seems a small effect but it is significant."
However, two-thirds of primaries said that they did not have enough money to spend on books after staffing costs and more than half of heads said ICT was a higher priority.
Mick Brookes, general secretary of the National Association of Head Teachers, said that 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."