Existing research reveals a robust relationship between self-reported print exposure and long-term literacy development, yet few studies have demonstrated how reading skills change as children read a book in the short term. In this study, 50 children (mean age 9.7 years, SD = 0.8) took turns with a prerecorded narrator reading aloud a popular children's novel, producing 6,092 oral reading responses over 1,093 book passages. Each oral reading response was evaluated by a speech engine that calculated words-correct-per-minute (WCPM). Mixed effect models revealed that text level differences, between-individual differences, and within-individual variations explained 13%, 56% and 32% of variance in WCPM, respectively. On average, children started reading the book at about 93 WCPM, and they improved by 2.26 WCPM for every 10,000 words of book reading. Random effects showed that the standard deviation of the growth rate was 1.85 WCPM, suggesting substantial individual difference in growth rate. Implications for reading instruction and assessment were discussed.