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10-pun de Yomeru Ohanashi 1st grade is part of a series of books aimed at Japanese elementary school students. This part of the series is aimed at Japanese 1st graders, typically children aged about 6 years old. It is important to note that this series is not primarily meant for foreigners who learn Japanese, but was created with Japanese elementary students in mind. While this might sound like a downside at first, it is actually what makes this series so great.
Why? Because it is authentic Japanese. The Japanese used in textbooks is often not natural and feels forced. In the process of trying to make it easy for foreigners, many textbooks lose their authenticity. Books that were not made for foreigners don't face the same problem. They are written by Japanese, for Japanese, in Japanese. If so far all the Japanese you have seen is from your textbook, you are missing out. If you are at around the N4 level, this book could very well be your first touch point with real Japanese.