THIS REVIEW CONTAINS SPOILERS AND SPOILERS-FREE ZONES. BE WARNED.

First of all, I should say that I've only read the first three volumes of the light novel and some chapters of the manga. This review will only cover the first two volumes at all.

SPOILER-FREE ZONE:

"Due to a single traitor, the game has begun. Those that wish to escape must abide by the terms and reach the end. Now, let this fun Demise Game begin―"

Since the sinopsis, Shuuen no Shiori (or the western name, Bookmark of Demise) begins with the rules of a so called survival horror "game". Just like any other story of this sub-genre, the rules are made to create some type of consistency between reality and fiction, trying to make characters don't kill each other without consequences.
The Book of Demise contains the description of all urban legends ever known and the legend written on the page where the Bookmark of Demise is set will turn real.
Our four protagonists, Boy A, Girl B, Boy C and Girl D (also known as A-ya, B-ko, C-ta and D-ne) are enthusiasts with occult themed things and got really interested with the idea of taking the Book on their hands. On the process of summoning the Book, they get stuck in a game of demise, where one of the participants is a traitor and an assassin. The game will only finish if the traitor die, else, everyone of them will die.

The series, just like KagePro, started as a vocaloid series, having lots of songs with important plot keys that, sometimes, are missed in the novel. The novels are REALLY better than the manga (the manga actually cut almost everything that made the novels great).

I should say that the story is nothing new, but is done so greatly (especially at the second volume) that it really catchs you.

SPOILER ZONE

I really can't say what I liked on the series without giving some spoilers of the surprising and anti-climatic final events of the second volume. I should say that I really love well done anti-climatic endings and the final chapters of the second volume totally nailed it, at least, in my opnion.

Without further explanation, what I really like in this series is that it REALLY remembers you that all the things that happened to our protagonists looks like fiction. It is like the rules for their game was written by some B-grade novelist (and this is reality). When they finally understood who is the "fox", I could hear my mind blowing. Before everything is brought to screen, I was really mad at the bad writing with lots of plot holes and mediocre situations, but when A-ya's gives his final veredict (that he, the character, was also bored with all the unrealistic setting) everything begin to fall into place.

The plot holes, the mediocre written situations... Every writing tool is actually... A foreshadow. It all make sense in the end. And that's all you need to know.