Many children fail to comprehend what they read because they do not monitor their understanding, which requires making accurate judgements of what they know and then employing repair strategies when comprehension fails. Relying on research from learning science and cognitive and developmental psychology, we developed the Word Knowledge e-Book (WKe-Book) to improve children's calibration of their word knowledge, strategy use, and word knowledge overall; skills which are associated with reading comprehension. The WKe-Book, which is read on a tablet computer, is a choose-your-own adventure book where choices require choosing between two rare words (e.g., cogitate vs. procrastinate). Depending on the word chosen, the story follows a different plot. There are also embedded comprehension questions where students receive immediate feedback with consequences for incorrect answers, such as being sent back to reread a few pages. In a randomized controlled trial, we tested whether students (N = 603 in 25 third through fifth grade classrooms in Arizona in the US) reading the WKe-Book would demonstrate improved word knowledge, strategy use, and word knowledge calibration. Classrooms were randomly assigned to read the WKe-Book immediately (treatment) or later (delayed-treatment control), and within classrooms, students were randomly assigned to either participate in a 15-minute weekly book club (book club treatment) or to read the WKe-Book independently with no book club (no book club control). Results revealed a significant treatment effect of the WKe-Book on students' word knowledge, word knowledge calibration, and strategy use, which predicted student performance on standardized reading comprehension and vocabulary measures. The effects were greater for students who participated in weekly book clubs compared to students in the no book club control. These findings suggest that the affordances offered by technology, which are unavailable in paper-based books, can support students' development of metacognition, including word knowledge calibration, strategy use, and word learning skills.