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10-pun de Yomeru Monogatari - 3rd Grade is part of a series of books by Gakken aimed at Japanese 3rd graders, typically children aged about 8 years old. Kanji Japanese 3rd graders are supposed to be able to read do not have furigana, making it more challenging than the 1st and 2nd grade. But don't worry: Most kanji still have furigana, just not all of them. The overall level also increased, and is appropriate for learners at around the N3 level.
As all books in this series, this book was originally not intended for foreign language learners, but for Japanese kids. Many language learners use this to their advantage to experience authentic Japanese, something that conventional textbooks do not offer.