Despite much attention having been paid to foreign language (FL) reading and related variables such as self-efficacy, relatively few studies have focused on young learners, and even fewer on storybooks, arguably the commonest genre read by children worldwide for language learning. As FL learning commences at a young age in many contexts globally, it is of importance to examine how FL children's perceptions of storybooks and reading self-efficacy relate to their proficiency. In the present study, 96 Grade 5 English-as-a-foreign-language (EFL) learners in Hong Kong completed a questionnaire (measuring their perceived value in English storybooks and English reading self-efficacy) and various proficiency tests (vocabulary size, word reading, reading fluency, and reading comprehension). Drawing on Expectancy-Value Theory (EVT), the interrelationships between the variables were analyzed using hierarchical regression and mediation. The findings revealed that self-efficacy was a significant positive predictor of all aspects of EFL proficiency while value was predictive of vocabulary and word reading only. Self-efficacy was also found to be a significant mediator between value and some areas of proficiency investigated. The study advances our understanding of EVT in which value and self-efficacy seem to have stronger predictive power of lower-order aspects of proficiency (vocabulary size and word reading) than a higher-order aspect (reading comprehension). The article ends with a brief discussion of the implications of the findings.