An extremely interesting report by USAID (US Agency for International Development).
In many countries, the books available in the primary grades are far too difficult for young children to read. They are eager to learn, but they may have limited vocabulary and the language of instruction frequently is not their mother tongue. For these children, learning to read is an enormous challenge. They will learn to read only if the instruction and materials are designed for their level of skills upon entry. Materials that support and engage children during classroom teaching play a key role in ensuring that they learn to read.
Children need books that they can read and share with parents, family, and friends. This paper examines ways to create texts in order to provide a continuum of reading levels for the primary grades. Using these methods, texts are calibrated to start at an easy level and gradually become more difficult so that children can access books they can read from the earliest weeks of first grade onward.
Click on this link to read more: https://allchildrenreading.org/wp-content/uploads/2019/07/Books-that-children-can-read-Decodable-books-and-book-leveling.pdf