I picked this and one other book up from a thrift shop on impulse. The Demon Samurai starts off like some sort of psychological horror story, as American filmmaker Mike Kirk heads to Japan to film his latest monster movie. Mike has problems. He's had three failed marriages, and he picks up any random pretty lady he sees, while spurning the affections of Mari, who works as sort of his assistant and genuinely cares for him. It isn't that Mike doesn't care for her, he simply is unable to open himself up emotionally to her because of his problems. His more immediate problem is that Mari caught the eye of some twisted samurai spirit, and the spirit isn't too happy about her being in love with a boozy, flirty foreigner. So he sets out to possess various people and infiltrate Mike's dreams to. . . I'm not sure. He keeps killing him in the dreams, but it doesn't kill him in real life. Is he trying to drive him mad, maybe just scare him into leaving home.
That was working pretty well, and then huge monsters begin to emerge from the film the crew is shooting for the movie, and begin laying waste to the set. Suddenly its a kaiju flick, complete with ineffectual military efforts. This also leads to a final chapter where an old sennin, sort of a mage/holy man type, has to explain everything to the rest of the cast (except Mike, who vanishes in any recognizable form roughly halfway through the book). So the first half of the book is a little clunky, there were parts that didn't make total sense, but there was a sense of momentum and a lot of tension to the whole thing. A guy fighting himself, but also some other force he doesn't understand at all. Then it falls completely off the rails after that because the horrors become much more literal.