This is a very hard book to describe, for a number of reasons. First, it's very, very layered. Second, it's challenging to balance useful description and not giving away spoilers. I'm going to do my best to describe the (physical) book, the experience of reading it, and the most obvious summary; but not in that order.
So I need to start with some terminology before I can talk about this book. First, the physical, real life artifact that you read is called simply S. and it is written by Doug Drost. S. is a slip-covered hardback volume, and within that slip-cover is a novel titled The Ship of Theseus, by the mysterious and notoriously private V.M. Straka. SOT is a fictional novel written around 1949 (3 years after Straka's death) and the particular copy that you have in your hands is one stolen from a high school library circa year 2000 by a 16 year old named Eric.
With me so far?
Eric goes on to become a Straka scholar, and leaves his underlined and benoted copy of SOT in the college library one day after leaving quite hastily. It is found by a young undergrad named Jen who works at the library. She leaves it for him to find again, but not before reading a couple chapters and making some notes in the margins. This continues for months as Eric and Jen leave layers of notes to each other in the margins of the work, while finding themselves caught up in a decades-old mystery that lures them deeper and deeper into the world of Straka.
Still with me? Good, because I didn't even mention the book's translator F.X. Caldera who is present throughout the original work in the form of extensive footnotes, and whose relationship with Straka becomes another plot line entirely as we proceed. Caldera will serve as one of a variety of unreliable narrators on our journey. I love unreliable narrators (they make you think) which is good because I'm pretty sure there are 5 in this book, simultaneously.
So now we have our cast. Straka - a prolific radical writer of the 20s, 30s, and 40s whose works inspired many of the pro-labor and anti-fascist movements of the day but who took great pains to never reveal his true identity. Caldera - the translator of Straka's works for decades who struggled to protect Straka's identity (and claimed to not know it) while acting as the caretaker of his work and legacy. Eric - the modern day scholar with a sketchy academic history struggling to resolve the age-old question of Straka's identity to make his career. Jen - the disillusioned and naive woman, months away from graduating and looking for control in her life.
And that's just in the margins.
The book itself is gorgeously produced. The illusion of the book, as a well-loved and abused copy of SOT is taken to ridiculous and wonderful extremes. The inside covers are strained, the pages are yellowed at the edges. The markings from the original library are stamped on the inside covers. Looking for a credit for Dorst or an actual copyright date? Better get a magnifying glass, it's very well hidden. Throughout the book are hundreds of margin notes by Jen & Eric in a variety of colors (and obviously, different handwriting), underlined and circled passages, little doodles and the like. I think I counted 3 pages that are just SOT and nothing else. Stuffed between the pages are a variety of artifacts that Jen & Eric left for each other as they worked together. Postcards, handwritten letters, newspaper clippings. I truly cannot imagine what it must have cost to make this book.The people who handled their graphic design work get a lot of credit in interviews, and rightly so.
The story is gripping. I found myself totally immersed in the world. It demands your attention, to track the story in SOT, the "real-world" people and events that it is an analogy for, the discussions in the margins and the footnotes, the happenings in J&E lives and how they relate to everything else. There's a lot going on, and the nature of the marginalia is that you get it out of order. On one page you might have notes from months apart between Jen & Eric, alluding to events that you don't have any knowledge of yet (maybe ever), or referencing footnotes 8 chapters ahead in the book. You'll want to read where you can spread the book out a bit. Keep a notebook by your side. Get involved. It's rewarding.
This is the book that I was hoping a certain other "novel-with-ephemera stuffed into it and a promise of mystery" that I reviewed on this site several years ago would be. That one was a disappointment, this one was exactly what I wanted. A fun read that left a lot of things up to me, that demanded my attention, and rewarded me for everything I put into it. If that sound interesting, I recommend it, and you should read it quickly because I need someone to talk to about it, dammit. It's been a week and I'm still spending some time most days trying to decipher a code or three or reading theories online. And learning how to use PGP so I can... nevermind. I've said too much already. They are watching and it is raining.
 J.J. Abrams gets a credit there too, because it was his brainchild.
 Well. I was rewarded. Your mileage may well vary. :)
 No, I'm not linking to it. I'm lazy and I don't want to accidentally read the review again. I think I was quite mean.