Siyavula is known as a pioneer developer of high-quality free digital maths and science textbooks to address resource gaps and disadvantage in South African schools. This case study identifies the success factors which could be replicated in other contexts. Siyavula has developed expertise in digital developing, editing and improving maths and science workbooks and teachers' guides and distributing them in multiple digital and mobile formats for free. In 2013 the government took the free texts and sponsored the printing and distribution of c500K copies of Grade 4-6 titles, saving the government approximately USD$83.5 Million for each of the 12 books (student workbook and teacher guide in both English and Afrikaans). The collaborative authoring system is identified as instrumental to the success of the project to address under-resourced schools, through a combination of personal (attitudinal), technical (online systems) and social resources (volunteers and stakeholders). Siyavula leaves a legacy of multi-stakeholder volunteer text-books sprints where an intense face-to-face experience provides the ground-work for constructive online authoring inclusive of diverse stakeholder input across different roles and ranks. Collaborative authoring advances curriculum and pedagogy sharing, expertise and capacity building. Collaborative authoring systems are found to have potential in many under-resourced school contexts not only for school texts, but also for early reading, multi-lingual and culturally appropriate book adaptations.