As major sources of information, curriculum materials (CMs) have generally served teachers and students by providing instructional and learning support. The impact of CMs on teachers' pedagogical practice and learners' engagement and success had been fairly documented. However, just as the use of CMs is influenced by teachers' orientations and learners' identity with the subject matter, the same can be said for textbook writers who attend, interpret and respond to curriculum content and standards. Guided by Dietiker, Males, Amador and Earnest (2018)'s curriculum noticing construct, this study examined the epistemological features of Philippine secondary school mathematics textbooks of the Department of Education. Four textbooks, one from each grade level, were analyzed via a directed qualitative content analysis. Following the inductive approach, significant statements from the texts were culled and organized through coding, categorizing and abstraction using a repertory grid. Member checking was done to validate the findings of the study. Interestingly, the study afforded the development of the Onion Model of Curricular Noticing Dynamics in Mathematics Textbooks. The model reveals how CM writers frame the mathematics curriculum in the design and development of textbooks. Specifically, the encountering frame lays emphasis on their interaction with the mandated curriculum through an examination of curricular elements. In the sense-making frame, writers reflect on four curricular questions leading to an understanding of the curriculum and the context in which it operates. Finally, the operationalizing frame captures the unique moves observed by textbook writers in developing the materials. The emerged model can serve as a valuable means in achieving heightened consciousness among textbook writers as they develop materials that promote teacher learning and respond to learners' needs, interests and orientations.