In the modern urban context, there exists a hidden group called "Echoers" - they are the existence of the deceased who return to the world due to strong obsessions. They maintain the appearance of their lifetime, but will gradually dissipate under certain conditions. There is a special organization called "The Ferryman" who is responsible for guiding these responders to fulfill their wishes and return to the other side.
This book is so good. I will regret not reading it. It's so imaginative and the plot is exciting. I recommend it!