To explore whether different balances of fiction/non-fiction reading and challenge might help explain differences in reading achievement between genders, data on 45,670 pupils who independently read over 3 million books were analysed. Moderate (rather than high or low) levels of challenge were positively associated with achievement gain, but non-fiction read was generally more challenging than fiction. Non-fiction reading was negatively correlated with successful comprehension and reading achievement gain. Overall, boys appeared to read less than girls, and proportionately more non-fiction, but this less carefully--especially in the higher grades--and had lower reading achievement. Differences between classrooms in promoting successful comprehension of non-fiction were evident, suggesting intervention could improve achievement. Implications for research and practice are explored. (Contains 2 figures and 5 tables.)