The Death of Ivan Ilyich by Leo Tolstoy. It's short, only a little over 100 pages, and mundane in its story (spoiler alert it's about a guy who dies) but very interesting. If you're like me and think too much about death, I recommend it. Very sobering and honest examination of the subject.
Next is spoilered because it's about a weird fanfic and not everyone here is about about weird fanfics.
Fallout: Equestria by Kkat. A crossover fic of MLP and Fallout. I read it on a whim as I've never been involved with the brony fandom or fanfics in general. My journey with FoE began as a bored hunt for cringe but evolved into an appreciation for just how much effort went into this story. It's extraordinarily well written and BIG, about 620k words. That makes it one of the longest works of derived fiction ever published, slightly longer than the Old Testament.
Quick synopsis. Fo:E is primarily an MLP fic set in a Fallout inspired universe. 200 years after a brutal war between ponies and zebras culminated in global devastation, LittlePip the humble and diminutive PipBuck repairpony must venture beyond the safety of her stable and brave the wastes of post-apocalyptic Equestria. What will she find? Some real fucked up shit.
My thoughts. As I said before, it's a well written story. The pacing fits the length of the narrative. Characters are continually developed. LittlePip is a fun ride so far as main characters go. Her flaws and realizations and strokes of genius are all entertaining. The low points of the story hit hard without being so depressing that you put the book down. Heavy subjects like addiction and the various moral dilemmas that come with any good Fallout thing are handled with dignity and richness that's almost alarming. There is cringe, either weird out of place dialogue that I'm pretty sure is a reference to the cartoon or the semi-frequent horniness of the protag. That's not too rampant but part of the territory, if you can't laugh along at the absurdity then why tf you reading MLP fanfics for my watermelon addicted friend? Anyway it's still an enjoyable read for the casual non-brony provided you're set for a long ride
I'm not sure why exactly Kkat decided to distill their genius into a fucking 620k word MLP fanfic, but y'know what? I'm glad they did.