@AnthoDerv
I was looking through my bookshelves.
My wife used "Big Fat Cat" (Mukoyama Takahiko) readers for Japanese.
There's a short mildly amusing story with furigana, then a notes section explaining the grammar etc. in detail. almost sentence by sentence.
To me the English reads normally. not esp. adult though, but engaging enough. It feels intermediate. not beginner, not advanced.
Can't really comment on the grammar explanations but it looks ok.

@AnthoDerv
or the lifestyle section of The Gaurdian? Tim Dowling's columns?
(Turn on adblock and turn off Javascript for a better experience though)

I couldn't think of any short form writers more recent than James Thurber. (that weren't genre)

@AnthoDerv
Thinking of my own studies (Japanese not English) have you considered magazines? On a subject that the learner knows or is interested in. The articles tend to be short and illustrated. The vocab personally relevant.
Children's news magazines are also good for current events. The language and structure although simpler is 'authentic'.
I believe the app Libby is used by some public libraries in Japan, who may carry English magazines like 'The Week Junior' for free.