I'm in the Bookstore, That's the Beginning

In the second-hand bookstore on a rainy night, a customer in black robe came. He didn't buy a book, only drank a cup of hot tea, leaving a puddle of black mud that couldn't be wiped off, then turned around and disappeared into the rain. I chased him out, but stepped into another world. There are black-smoking factories, running carriages, high-ranking nobles, and civilians struggling in the mud. But no one told me that this world contains secrets that cannot be seen, sounds that cannot be heard, and restricted areas that cannot be set foot in. Some people became monsters because they understood half a page of an old book, and some people gained power because they heard a whisper. I thought I was a passer-by who broke in unexpectedly, a variable that changed everything. It wasn't until much later that I understood, My destiny has already been written.