Let's discuss unique books. What is the first one that comes to your mind? The uniqueness can be about the book's plot, structure, ideas, themes, topics etc.
For me such a book is Cat's Cradle by Kurt Vonnegut Jr. It's been a while since I've read it but I remember how mindblowing it was for me. Every time I thought it couldn't surprise me more, it actually did. There's a bit of everything there: peculiar characters, a substance called ice-nine that can freeze water on contact (as humans are around 60% water, it can instantly kill them), a fictional religion called Bokononism based on the idea that human life has no absolute truth or meaning, a tiny island nation with its own bizarre world. All of these come together to show us a satire of the absurdity of humanity.
As for a unique book structure, I have to say it's Cain's Jawbone by Edward Mathers. It's a kind of puzzle book where pages are not in order and you need to arrange them in order to guess whodunnit. I haven't finished this one yet but it's a cool challenge.
Structure-wise, probably House of Leaves. It uses appendices and several categories of footnotes extensively, to the point that the footnotes begin to dominate the pages (putting it lightly). There's so much flipping back and forth that an onlooker would assume you're reading a choose-your-own-adventure book.
Content-wise, it's hard to choose. I'll say The Illuminatus Trilogy, which is a semi-absurdist study of authority and human nature, through the lens of a labyrinthine web of conspiracy theories, jumping between at least a dozen character perspectives.
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