Um, I can’t really answer that I’m afraid, as my approach to grammar is rather random - I just read native content and look up whatever looks like unknown grammar. From what I’ve seen so far - and bear in mind, it’s only a few pages - the language is straightforward, nothing flowery or old-fashioned, no overly complicated descriptions or weird grammar constructs. There is no furigana (with few exceptions), in case that matters to you. It looks about the level of difficulty of Before the coffee gets cold, which is very approachable.
I couldn’t read a sample on Bookwalker because it’s on an all-you-can-read offer (I’m sure there must be a way that I’m missing), but there is one on Booklive if you want to give it a try.
Edit to add: For what it’s worth, it’s been classified as Level 38 (Intermediate, ~JLPT N2) by one person on Natively. By comparison, コーヒーが冷めないうちに is graded as Level 30 (Intermediate, ~JLPT N2). These are all all subjective comparative ratings by readers as far as I know.
Edit 2: I looked up all the books mentioned here on Natively. Some weren’t listed, but the other ones were all in the 30s, with only 硝子のハンマー being higher than 三毛猫ホームズシリーズ at 39. Suspect X was a 33.