As for books on aozora: some created a list ranked by wanikani level and kanji coverage (80% / 85% / 90% / 95%)
There is two stories from the shônen tantei dan series at the top. I can’t tell about grammar but at least you can choose something that suits your kanji level 
(and sorry, in case you already know about the list.)
Apart from that, I’m reading a story by Mimei Ogawa right now. He was an author of short stories, children’s stories, and fairy tales.