Evidence suggests that books in the childhood home impact children’s educational and occupational attainments. Previous research finds this ‘book effect’ after conditioning on parental socio-economic status and national context. Human and cultural capital theories offer plausible explanations for this book effect. However, mechanisms underlying the book effect remain empirically illusory. Drawing on previous findings, the authors develop theoretical arguments that human and cultural capital are not the entire story behind the book effect. Designing a formal model of all these processes, the authors test their claims using immigration, language and country-context to adjudicate between mechanisms hypothesized by the theories. Using CILS4EU data in four countries and structural equation modeling to test these theories’ models against the data, the authors find the direct effect of books does not differ between native and immigrant adolescents in predicting 9th grade language aptitude scores. Moreover, this effect does not vary much by country. Reading habits and primary language spoken at home explain only part of the effect suggesting the presence of books measures something operating independently and in addition to human and cultural capital. The authors suggest books create opportunities for children whose parents are unlikely to cultivate scholarly values on average.