It’s touted as the greatest Japanese science fiction novel of all time with its glow-in-the dark cover, and I’d like to be able to agree, or for that matter, disagree with that fact. However, my knowledge of Japanese science fiction tends to be limited to anime, and I don’t think that’s a fair thing to judge SF books by. So what I’m going to say is that while I can’t have an opinion on whether or not 10 Billion Days and 100 Billion Nights by Ryu Mitsuse was the greatest, or that I’m not entirely sure it’s science fiction, I can say that I know that the book is Japanese… so… at least I can definitively take a stand there.
So, let’s start out with the reasons why someone would want to read this:
 1. Jesus is a super-cyborg assassin.
Right. I’m a bit at a loss of what to say, and I think it’s because I’m just genuinely not used to my science fiction taking historical characters and making them live weird double/triple/quadruple lives in different time periods. I’m also ill-equipped to deal with epic death battles between people like Siddhartha and Plato against Jesus, and having the Asura (generally thought of as a group dieties) personified by a little girl  who is actually fighting for humanity rather than destroying it… which even if you have just a very basic grasp of Hinduism, this ought to strike you as a bit odd. Kind of cool, but odd.
So, here’s the deal. When you buy this book, the description is about as enlightening as the ending is:

Ten billion days–that is how long it will take the philosopher Plato to determine the true systems of the world. One hundred billion nights–that is how far into the future he and Christ and Siddhartha will travel to witness the end of the world and also its fiery birth.

 If you’re baffled at the plot described above, then you and I are on about equal footing and I’ve actually read the book.
Here is the actual plot: Something called Shi (for non-Japanese speakers, this word means “death” in Japanese) is trying to destroy all life on the universe via a eon-spanning whispering war against a benevolent “god” who only shows up in the last few pages in order to say how sorry it was that it didn’t.. I don’t know… whisper hard enough or something. Shi sews the seeds of destruction in mankind through war, plague, and civil strife… and apparently Christianity. I’m not certain, but I’m relatively sure that this book is about how Christianity is going to destroy the universe.
…And the person against all of this is Asura, who is the main character despite not being introduced until halfway through the novel.
This thought, then, brings me to my chief complaint, which is that this book is organized in a very bizarre way. It’s completely impossible to tell what’s important, what’s just random description (which is more often than not), and what’s allegory. It’s also more than a little difficult to keep hold of a timeline as each character’s plot jumps around each other in haphazard directions and only converge in the final free-for-all that is the slow decay of society.
Strangely, rereading that paragraph has made me suddenly like the book…. which I find slightly disconcerting seeing as I felt almost angry when I finished it.
You need to appreciate the Japanese way of thinking in order to really find pleasure in this book. The lack of resolution is so poignant in this novel that it may leave the Western reader feeling out of sorts and more than a little befuddled. Western readers, I feel, are used to feeling satisfied at the end of a story. The Japanese are not. In a way, I find that it is a beautiful concept, if not a very Japanese one, and I enjoy it in most stories (particularly with Ryu Murakami).
I must concede, then, that I’ve steeped myself too deeply in Western sci-fi culture, because my dislike for a book I find interesting at a conceptual level lies solely in the structure of the novel and what I perceive to be gross misunderstandings of Greek philosophy and the New Testaments teachings.
On the other hand, it has a somewhat M. Night Shyamalan twist to moral concepts and philosophies that we deem peaceful (meaning that they lead to our ultimate destruction by the powers that be), so there is that… if only it wasn’t based in a somewhat skewed (read: Japanese) understanding of these ideas. In the end, it ultimately adheres to the Hindu concept of samsara, which is the cycle of life, death, and rebirth, which already makes it more interesting conceptually than most science-fiction novels out there.
I suppose that the best way to end this review must be to say that if any of this sounds interesting, you should absolutely read it. Just be prepared to struggle from time to time.