About 20 years ago, I read a book with a world where there was a special source of ink. It starts with a historical scribe found the source of ink when he ran out of ink and used it to depict the battle of his king. He then noticed that what he wrote happened on the battle field. When he wrote the battle successes of his king(that were a bit "modified from the truth") that actually played out in front of his eyes. When he wrote about successes of the opponent that actually happened in front of his eyes.
I believe the ink was the result of old dragon bodies turning into oil. And that the magic of the dragons got transferred into that oil. Resulting that what was written to become reality, even for a short while.
This led to the formation of a select group of people who knew of the ink, who used it to set up elaborate lives, grant themselves eternal youth, or whatever they managed to dream up/imagine.
The book itself takes place when the ink is running out and all the magical castles are crumbling because they need to be rewritten in the ink every X years to continue to exist.
There was the protagonist, another magician who had written himself wings who tried to steal the protagonists ink because his ink was as good as out, and he didn't want to lose his wings/castle/life.
I believe the protagonist was also meditating on a levitating rock at his introduction to the story. The protagonist used his stash of ink sparingly, which led to him still having enough to perform his own sparse actions of magic.
I believe there were 3 sisters, with two still in their realm and one sister trapped in our realm.(earth, modern day 90's I think)
Another detail I remember, put in spoiler not to ruin it for people.
Somehow the protagonist ends up in our world, ends up having to have sex with a woman in a mental institution(or somewhere else where there's guards) so their bodies are joined, so the sisters can pull her along with him out of the world. Right at the moment the guards break down the door they are vanished.
I believe there were 3 sisters could be more though, with two(or more) still in the realm with one sister trapped in our realm. but I'm really not sure.. it's been 20 years and I was 12 at the time, and no it was not a kids book :-P it inadvertently ended up in the kids section of the library
That's all I remember of the book.