Despite the proliferation of digital mediums such as documentary series, video essays, and science podcasts, popular science books are still the primary medium for promoting science to the public as an epistemic way to understanding and being aware of our natural world. That said, in the domain of systemic functional linguistics, there exists a dearth of studies investigating popular science books. Hence, this study aims to investigate the organization of popular science discourse from the perspective of Theme Progression. A 93,078-word corpus was collected and divided into two main science categories: hard and soft—three disciplines under each main category and six texts under each discipline. The analysis of the corpus followed a mixed-method design where a Theme-counting excel sheet, created by the researcher, was used to calculate the most occurring Theme patterns. The results of the analysis indicate that the hard science disciplines give a logical presentation of the scientific text that intends to unpack and explain the intricacy behind the scientific notion whereas the soft science disciplines focus on expository narrative to connect and relate the scientific text to the target reader. Given these writing behaviors between the two science categories, it is recommended that future SFL studies explore the potentially arising differences within the broader aspect of the popular science genre, for instance, to juxtapose books from articles in the way discoursal features (e.g., coherence and cohesion) are structured in the text.