Interactive app books are increasingly part of young children's literacy ecosystem. However, most previous studies examined buddy reading with traditional print books or CD-ROM books. Little is known about whether and how buddy reading with app books might be related to subsequent individual reading. To address this, informed by multimodal literacy and sociocultural theories, we investigated how 53 kindergarteners' (ages 5--6 years) buddy reading behaviors were related to their subsequent individual reading behaviors and comprehension outcomes while reading app books. Multivariate mixed response analysis yielded these findings: (1) buddy reading monitoring behaviors (asked questions, drew attention to book content, debated, or negotiated) were associated with higher inference/critical thinking and vocabulary meaning generation scores; (2) buddies who read in triads had lower individual-prompted retelling scores than buddies who read in dyads. The findings highlight the importance of promoting monitoring during buddy reading and paying attention to group size.