I read this book in 2007, it was written in English. The story revolves around a man who has been cursed (I think) to have a demon follow him.
The demon is normally invisible and the man is the only person who can see the demon. When the demon feeds or kills people he changes shape to a huge creature and becomes visible.
The demon is also invincible. I think I remember the man and demon talking about a time when the man tried to kill the demon after being cursed but the demon just came back.
It's quite a graphic book, not fit for children. It contained swearing, violence, sex, and possible drug use. I'm not sure if it was part of a series or just a single standalone book.
There are three scenes I can vaguely remember:
The man and the demon are in a bar. The man is playing pool against a bar regular who is apparently the best pool player in town. The man isn't too good but the invisible demon helps lead the balls into the pockets (insane shots like a ball curving 90 degrees). The regular gets mad that he lost. The man and the demon were playing pool to get money (for what I can't remember)
There was a scene where an old man is telling his friends about the night before: He wakes up in the middle of the night and tells his wife "The bear is out of the cave" or something similar. His wife just rolls over and says "You better put some ice on it". This story was alluding to the man wanting to have sex with his wife for the first time in ages, but she was not interested at all.
I remember a hospital being destroyed when the demon goes crazy and needs to eat. The love interest might have worked at the hospital.
I will add more details if and when I remember them.