There is a prevailing belief that second language (L2) textbooks should strive for authenticity, aiming to accurately and reliably represent natural language use. However, assessing the authenticity of language textbooks is not a straightforward task, as it requires both a comparison between instructional texts and real-world language use and a consideration of the comparison within the broader context of language development. This study employs a corpus-based approach to examine word use in 31 widely used commercial textbooks across different proficiency levels, which serve as the primary resource for teaching, learning, and assessment in Chinese language programs at US universities. Using hierarchical cluster analysis, Zipf's law analysis, and descriptive statistics, we found supportive evidence for the authenticity of word use in Chinese textbooks from a developmental perspective. Specifically, our findings reveal a nuanced gradation in the distribution of words across proficiency levels within these Chinese textbooks. Moreover, as proficiency levels increase, there is a discernible register shift in word use from spoken to written discourse. These observations carry significant implications for the utility of textbooks in Chinese language programs, particularly in the realms of instruction and assessment.