graded reader
noun [ C ] uk/ˌɡreɪ.dɪd ˈriː.dər/ us/ˌɡreɪ.dɪd ˈriː.dɚ/
分级系列读物(用于阅读教学,或作为学习外语者自己阅读之用)
one of a series of books of increasing levels of difficulty, used for teaching people to read, or to help them learn a foreign language by reading