I remember a story that I read at least 15 years ago (which is possibly older).
The point of the story was that the creatures of darkness (like vampires) were regularly beaten by humans, and therefore regularly come back under new appearance and power.
The plot is in a modern setting. The protagonists discover the new monster and how to kill them. I seem to remember that to kill them you must eat their heart (they remain alive as long as it is not eaten). I clearly remember a line where a protagonist says that it is better cooked than raw (though the story says that the taste of the heart is disgusting).
In the end they don't kill all the creatures, but they imply to one of them that they will be far easier to kill than vampires, since they found their weakness so easily (when it took at least hundreds of years to get rid of vampire).
Last but not least. I am french, I read it in french, and although I believe it is an english / american novel (from the tone of the book and what I recall of the setting) it might be a french book.
Thanks to all the people that could help find the title of this book.