Number of books at home is often associated with reading, mathematics, and science skills. Number of children's own books has rarely been considered in previous studies as a separate variable. The present study aimed to investigate relations between both of these indicators and cognitive and socio-emotional outcomes in secondary school students, while analysing mechanisms underlying relations of these indicators with outcomes. Based on cross-sectional data of a German sample of 407 secondary school students (M[subscript age] = 12.72 years, SD[subscript age] = 0.38, 51.1% girls) and using multiple linear regressions, the number of books at home was found to be related to students' cognitive skills, while the number of students' own books was mainly associated their socio-emotional skills. Relations between the number of books, the number of students' own books, and students' outcomes mostly did not change when students' and parental behaviors were considered. Thus, future studies are needed with the aim to investigate relations between the number of books and outcomes in a longitudinal manner and to explore further potential mediators behind these relations.