Although it has been argued that school libraries are important for supporting the reading engagement of pupils who receive free school meals, to date there has been little analysis of the extent to which use of school library spaces is related to these pupils' reading behaviors. We analyzed data from 6,264 children and young adults in the United Kingdom entitled to free school meals (FSM) who completed the 2019 National Literacy Trust Annual Literacy Survey. Our goals were to understand the extent to which these pupils' engagement with reading and writing were related to access to or use of their school libraries and to understand some of the reasons why they do or do not use school libraries. We found their enjoyment of both reading and writing, their confidence in their own abilities, and the frequency with which they read or wrote for pleasure outside of school was significantly higher for those pupils eligible for FSM who used their school libraries relative to both those who did not use their school libraries and those who had no school library. Consistent with this finding, children eligible for FSM who used their school library engaged with a greater diversity of reading material and writing than those who were not school library users. Pupils who had access to welcoming, well-equipped libraries with books well-matched to students' interests used the libraries for those reasons and because, for many, it was a safe haven. Many non-users either had no school library or perceived the library as poorly equipped, unwelcoming, or uninteresting. We argue that effective school libraries can be a significant resource in supporting engagement with self-motivated literacy practices in children from low-income families.