An old one, but I read "The Brothers Karamazov" by Dostoevsky. I was expecting it to be dense and difficult to read, but I found it incredibly engaging - easily the best book that I have read, ever.
I think my favorite was Crime and Punishment. Dostoevsky is very good; I think the biggest challenge is just getting used to Russian names, which is exacerbated by the typically large number of characters. But if you're willing to brave that, I don't think I outright disliked anything of his I read.
Loved discussing this one as part of a book club, drew so many more insights out of the book by hearing what others gleaned from it.
+1 I recently read a The Idiot as well, another incredible work of his.
Sweet! Also, I think I follow you on Github! Your context tree weighting blog post is and has perfect depth. Thanks for writing it!