Finch takes place about a century after the events in Shriek: An Afterword, and is intricately intertwined with both that book and its predecessor, City of Saints and Madmen (in fact, the main character here even picks up and reads sections of Shriek at one point, and we learn that not all the characters from earlier times have entirely passed from the stage).
John Finch (not his real name) is an Ambergrisian detective, following orders from the fungal overlords who now control the city and who seem to be getting closer and closer to some new phase of their occupation and domination. Charged with investigating a very strange double murder, Finch finds himself pulled in increasingly strange and dangerous directions by what he begins to uncover. As his partner succumbs to a nasty and painful parasitical infestation, and shadowy characters with ties to the mysterious rebel powers and outside forces make themselves known, Finch must try to save what he can of his city, and himself.
Dark, brutal, and complex, this book shows off VanderMeer's skill at universe-building at its best, even though it might try to show off a few too many different narrative styles at once. Overall, if you like VanderMeer's works, give it a shot.