While searching for books that are simple enough for me to read in Japanese, I’ve been musing about the attributes that make these suitable for language learners.
If starting from scratch you need repetitious text with obviously illustrated nouns. This describes some of the books I purchased. The downside of the simplest books is that they become an illustrated list of nouns (or adjectives like colours), and therefore have no narrative.
In Japanese you have the added complication of the writing system. Beginner books use hiragana only. Then there are some that have katakana with hiragana transliterations. Then there are a few that use both the alphabets without guides. At the next level kanji are included with hiragana guides. The level of support for kanji varies.