English classrooms in Sweden host students of mixed abilities, all entitled to adapted education. Despite this, English language teaching, unlike other subjects, primarily relies on course books, serving as both learning tools and syllabus references. However, previous research has highlighted shortcomings in these books, such as insufficient vocabulary exercises and gaps in alignment with the syllabus. Although research and the Swedish National Agency of Education advocate for less reliance on course books and more communicative teaching, many teachers still find them useful. This study aims to explore the extent to which these course books allow for differentiation, to meet the needs of all students. The study employed triangulation method, by mixing a content analysis of 15 course books and interviews with six English teachers. The investigation's primary conclusions are that there are not many opportunities for differentiation within the books, but teachers seem to make do with alternative resources, adapting the books tasks and giving students varying degrees of support. While some differentiation is possible within course books, it may not fully address the needs of students of mixed-ability classrooms. Therefore, it could be concluded that the current approach may be insufficient to ensure that every student's needs are met, potentially limiting their language acquisition opportunities.