動物園は大さわぎ! 動物と話せる少女リリアーネ

This is a children's book for kids ages 8+, translated from German into Japanese (no English translation, as far as I can tell). Its original German title is Liliane Susewind – Mit Elefanten spricht man nicht! by Tanya Stewner. If you like stories about animals, you may find this to be a good choice as your first "real" book in Japanese, similar in difficulty to the Magic Tree House books.

The story is about a little girl who can speak to animals, and how this super power has made her a social outcast and the target of school bullies (which is frankly absurd, since every child I've ever met would think talking to animals was the coolest thing in the world — but whatever, that's kidlit for you). Along the way, she makes a new friend and takes up a mission to help a sad elephant at the zoo. The story's not bad, but if you don't like children's literature this will probably bore you.

A separate series by the same author about the same characters but written for younger readers (ages 6+) exists, so if this book is too hard for you but you're still looking for something like this to get started reading Japanese books, you can try the younger series instead. The book I read from the younger series was 動物と話せる少女リリアーネはじめてのものがたり しあわせの黒いねこ, which I also reviewed on this website. If you're shopping for this series and not sure how to distinguish between the two, the books for younger kids are white hardcovers only about 120 pages long, while the books for older kids are colorful paperbacks over 200 pages long.