Although retrospective project reports are common in the materials development literature, accounts of textbook writing sessions are rare; so too are accounts of open textbook production. Open textbooks are learning resources that are free to use and oftentimes adapt by virtue of their copyright permissions. The authors used concurrent verbalization and interviews to document writing episodes while preparing their first book, an open textbook devised for corequisite technical writing courses. Corequisite designs pair content courses with explicit skill-building modules as a means to support underprepared learners in higher education in the United States. Qualitative content analysis of the data revealed how teaching and other praxis influenced the open textbook's composition: in the authors' applications of technical writing principles, pedagogical reasoning skills, and nonteaching work. The findings may encourage open textbook writers to exploit their established composing practices and knowledge bases to proceed with textbook production. In addition, the article highlights the usefulness of concurrent verbalization to textbook research and identifies the various materials development opportunities open textbook projects provide. It also contributes to the underresearched area of textbook production by exposing the complexities of open textbook development and how two novice authors negotiated them during writing episodes.