graded reader; limited-vocabulary reader
A book written with a controlled vocabulary, often for language learners. The term is technical and used in educational or linguistic contexts.
Graded readers for Japanese learners are increasing.
This limited-vocabulary reader is written using only beginner-level vocabulary.
多読 refers to extensive reading as a learning method, while 語彙制限本 is a specific type of book used in such methods.
Compound of 語彙 (vocabulary) + 制限 (restriction) + 本 (book). A modern technical term.