Reading is a fundamental skill that is critical for later academic success. Despite extensive research on its development, reading remains a challenging skill for students in the U.S. to master: recent data from the National Assessment of Education Progress showed that 65% of fourth-grade students score below the level of "proficient" in reading (National Assessment of Educational Progress, 2019). Of that percentage, students from minority or less affluent socioeconomic backgrounds obtained even lower scores. Given this need, research has predominantly focused on supporting reading development as a skill (National Reading Council, 1998). However, once a child develops competence in decoding, building expertise as a reader requires sustained engagement in the activity of reading, both for pleasure and as a resource for learning (Gambrell, Mazzoni, & Almasi, 2000; Taboada, Tonks, Wigfield, & Guthrie, 2009). National data affirms that supporting reading engagement is an important area to work on, as recent reports have found the amount of time that children read for fun has been dropping (Common Sense Media, 2014; National Assessment of Educational Progress & National Center for Education Statistics, 2012). Collectively, past research and national data points to a need for more research on how to nurture motivation and engagement to sustain a child's reading development into eventual expertise, both as an individual capacity and as a shared activity within a wider community. In this field of research on reading motivation and engagement, a key but under-explored area is the role of social motivation (Rueda, MacGillivray, Monzó, & Arzubiaga, 2000; Wigfield & Guthrie, 1997). Although the social context is underlined as central in learning theory (Rogoff, 1995; Vygotsky & Luria, 1978), the social aspects of motivation for reading engagement are not well elaborated in canonical reading motivation theories (Schiefele et al., 2012; Wentzel, 1996; Wigfield & Guthrie, 1997). More research is needed to understand the multidimensionality of social motivation and how it can be leveraged to nurture successful reading engagement, especially when it is situated in a social context. A parallel topic of interest to social motivation in reading is the recent development of educational technologies with social affordances. Past research has highlighted the capacities of social educational technologies for creating new, interest-driven opportunities for learning (Ito et al., 2012). However, the performance of these technologies to meet learning goals in classroom contexts varies widely. In particular, the ways that the social aspects of these technologies might best be leveraged to foster reading engagement require further study (Cheung & Slavin, 2012; Mangen & van der Weel, 2016; Weston, 2004). Investigating this topic would also contribute novel insights to reading motivation theory, particularly how the social dimension of motivation interacts with social affordances from digital tools. In the following dissertation, I address this gap in reading research with a multipronged approach to the broad questions of: how do learners socially engage around reading, given a social digital reading tool? My research leverages multiple methods to examine this question in the case of students from four elementary classrooms using one online reading exploration platform, Bookopolis. Bookopolis is aimed at elementary and middle schoolers, offering social affordances for reading engagement such as sending and receiving reading recommendations and sharing digital reader profiles. In the sections to follow, I present three studies that each take a different methodological approach, revealing unique but complementary insights to understand the phenomenon of social reading engagement around Bookopolis. In Chapter 1, I use social network analysis to examine reading engagement in the form of social reading recommendations on Bookopolis. Specifically, the ties in the network are defined as reading recommendations that are sent and taken up between peers. A stochastic actor-oriented model (SAOM) is implemented to examine what drives the evolution of the social network arising from reading recommendation uptake, as well as reading behavior within the network, for one classroom of 28 students. SAOM allows for the creation of models that simultaneously account for the evolution of reading behavior and the evolution of the ties within the social network over time (Snijders, van de Bunt, & Steglich, 2010). The study results indicate that students tended to send fewer reading recommendations over time, as well as read more books and a wider breadth of books over time. The analysis also found that neither peer influence nor selection effects significantly drove these changes in reading behavior or network ties. Rather, network ties were driven by one degree-related effect as well as structural tendencies of a social network, specifically reciprocity (the tendency to reciprocate received ties) and transitivity (the tendency to have ties with friends of friends). The findings suggest that students tend to have recommendation uptake ties that are reciprocal and within friend groups, as well as ties that are more selective rather than broadcast. The overall study offers novel insights as the first application of SAOM to a social reading recommendation network created by young learners in an online context. In Chapter 2, I use mixed methods to characterize a user typology of the unique ways that students socially engage on Bookopolis, across the sample of 122 students from four classrooms. Guided by questions of what are the unique types of engagement exhibited by students and how/why they used their preferred features on Bookopolis, I leveraged the combined strengths of cluster, descriptive, and qualitative analysis to identify and characterize four types of users: Reviewing Receivers, Collecting Senders, Minimal Users, and All-Rounders. While students of each user type overlapped in some aspects, interviews revealed that each group described distinct preferences for engaging in specific features, which was correspondingly reflected in distinct quantitative activity on the platform. The methodological approach in Chapter 2 demonstrates an example of how mixed methods can be used to characterize user typologies while centering participant voice in the data. Further, the findings of this research showcase how students can meaningfully engage in a range of ways using digital reading tools such as Bookopolis. These insights have implications how social digital tools can be better designed or implemented to encourage different styles of social reading engagement. Finally, in Chapter 3, I take a qualitative approach to investigate students' motivations for socially engaging with their peers around reading, given Bookopolis as a tool. Based on artifact-based, semi-structured interviews with 43 students across four classrooms, the qualitative analysis surfaced new facets of social reading motivation around the themes of helping others, sharing interests, competing, and using social virtual affordances for engagement. These findings built upon past conceptualizations of social motivation for reading engagement, beyond the idea that students engaged to help peers with schoolwork or discuss reading materials (Schiefele et al., 2012; Wentzel, 1996; Wigfield & Guthrie, 1997). The analysis found that students were motivated to engage in prosocial behaviors of sharing and curating reading recommendations with their peers. Students were also motivated to socially engage around common interests, particularly in the context of reading, in addition to engaging to manifest new common interests. The analysis further presents unique insights on what social virtual affordances students valued and why, such as maintaining digital profiles as readers or sharing online recommendations to engage with their peers around reading. Overall, this study contributes new dimensions to the theory of social motivation for reading, specifically how motivation can be augmented by the affordances of social digital reading technology. Together, the three studies of my dissertation contribute unique and complementary insights to the question of how young learners socially engage around reading given a social digital reading tool such as Bookopolis. From a methodological standpoint, the mix of quantitative and qualitative approaches helps address the tradeoffs of each selected method, while also providing depth to the findings that surfaced from each study. I elaborate the ways that each study builds upon the other in Introduction, in addition to the Discussion section of each presented study. Furthermore, the three studies together offer both practical and theoretical contributions to the questions of social engagement around reading. Practical contributions from Chapter 2 offer insights as to how and why an educational digital tool such as Bookopolis can be designed or used to support classroom-based social reading engagement. The theoretical contributions of Chapters 1 and 3 elucidate how socially engaged reading behavior manifests as a social network and as a form of reading motivation. Collectively, the range of approaches presented in my dissertation identifies novel, multifaceted insights on the phenomenon of social engagement around reading as it interfaces with technology. These findings inform how the design and implementation of social educational technologies can be leveraged to nurture sustained reading motivation and engagement.