I think any attempt to describe the story would not do the novel justice, so I won't try. Instead, I'll go over what the novel represents.
This series is a "supernatural" psychological mystery, with minor touches of unvocalized love/romance.
(Note: I put supernatural in quotes as it is not the typical concept of supernatural.)
The first volume is more supernatural mystery. It's also a bit of a mindf*ck at first. Because of this initial confusion, the beginning of the first volume may be a bit difficult to get through. However, once you start getting a grip on the situation and understand what's going on, it will becoming much more enjoyable and become interesting quickly. Personally, I ended up reading to the end non-stop. The supernatural environment and circumstances of the first volume make it truly epic.
After the first volume, there's a rather severe shift in the nature of the series. As a result of the resolution of volume 1, the epicness ends and a bit more normality returns to the series. This isn't necessarily a bad thing as it opens up new situations and interactions not possible in the first volume. At the same time, it retains its can't-put-it-down awesomeness.
Starting from volume 2, the series gets heavier on the psychological mystery components. The mysteries are never simple and attempting to figure out what's going on yourself can be quite challenging and mindbending. Regardless, it remains interesting, especially with the character interactions of the two main characters.
All of in all, I give this series a 10/10. I can't wait for volume 4 to be translated.