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

I used this book as a bridge between graded readers and native material. For language learning purposes, it's great because the text is gigantic so it's easy to read the furigana, the sentences are simple, and the illustrations help you decipher what's going on if you're stumped. The book was translated into Japanese from German (original title: Liliane Susewind – Schwarze Kater bringen Glück by Tanya Stewner), and be aware there's no English version anywhere, as far as I can tell.

I actually enjoyed the story — it's about a little girl who uses her ability to talk to animals to help them solve their problems. In this book, she befriends a black cat who isolates himself from others because he thinks he brings bad luck wherever he goes. There's another cat who speaks in keigo (don't worry, it's not too hard) and is so full of herself she's a riot to read; A++ best character in the series.

As a final note, the Liliane Susewind books come in two series. This book is part of the series for younger readers, ages 6 and up, and each book is only 120 pages long; these are self contained side-stories separate from the series for older kids. If you can finish this book, you can move on to the series for ages 8 and up, which is slightly more challenging since those books are longer (200+ pages) and the text is smaller (but still larger than the typical Japanese novel, and still has full furigana). The books share a lot of vocabulary, so that makes it significantly easier. The ages 8 and up series starts with the book 動物と話せる少女リリアーネ1 動物園は大さわぎ！