Materials development is increasingly recognized as a valuable venue for narrowing the research-practice gap. This qualitative case study explores how four middle school teachers and three university researchers collaborated in a community of practice (CoP) to write textbooks by highlighting the interplay between their collaborative relationships and actions. Data were collected from several sources, including semi-structured interviews, observations, and artifacts. The findings reveal that this textbook writing CoP was characterized by the interplay of joint enterprise, mutual engagement, and a shared repertoire; mutual engagement, featuring dynamic interaction between the participants' collaborative relationships and actions, played a pivotal role. In light of a nuanced interpretation of teacher-researcher collaboration in this CoP, we foreground the role of mutual engagement and the factors which influence it. The study concludes by highlighting the mutually reinforcing nature of the dynamic interplay between language teachers' collaborative relationships and actions in bridging the research-practice gap in communities.