Third to fifth graders read an interactive choose-your-own adventure e-Book. User logs recorded their reading behaviors and were used to investigate students' in-the-moment reading behaviors. Reader's standards of coherence, motivation, and reading strategies were hypothesized to relate to children's reading behaviors, such as time reading pages, answering embedded questions correctly, and making thoughtful decisions, and in turn to word knowledge gains. Structural equation models revealed that the more time students spent reading embedded questions, the more likely they were to answer the questions correctly, which in turn strongly predicted gains in word knowledge. The more time students spent reading text pages, the more likely they were to make good decisions. Additionally, student participation in a weekly book club, randomly assigned within classrooms, predicted stronger gains in word knowledge. Findings highlight the potential of e-Books to improve word knowledge, and that student user-logs offer another way to study reading comprehension in-the-moment.