Welcome to White Rabbit Japan!
10-pun de Yomeru Ohanashi 3rd grade is part of a series of books aimed at Japanese elementary school students. This part of the series is aimed at Japanese 3rd graders, typically children aged about 8 years old. Japanese kids are supposed to know 440 kanji at the end of the 3rd grade. This clearly shows in this book, as it contains a lot more kanji than the 1st grade and 2nd grade parts of this series. While the grammar in this book is still mostly at around the N4 level, vocabulary and kanji is more towards the N3 level.
As mentioned, this book is meant to be read by Japanese elementary school students and that's what differentiates this book from a regular textbook, what makes it authentic. For many language learners, this book could very well be the first source of authentic Japanese.