books that are hard to recommend but worth it?

I’m not looking for the usual easy to read and everyone loves it type of books

I’m more curious about those books that are a bit weird, slow, confusing, or even frustrating at times… but still worth reading in the end

the kind of book you hesitate to recommend because it’s not for everyone, but it did something interesting or different

what’s a book like that for you and what made it worth the effort?