The story is mostly about Savannah, the quaint coastal Georgia city. You could go so far as to call it a character of the book; its peculiar citizens and strange ways are the driving force behind the novel and the movie. But whereas in Eastwood's movie you can feel the strangeness, the book reads too much like a brochure: How quirky is Savannah? Why, here's some dry, lengthy historical background to show you. It's not unenjoyable, but the characters in this book too often talk like travel brochures, spitting out dates and events as if they've committed them to memory. Even the more colorful characters like the transvestite Lady Chablis (who is a real person and plays herself in the movie) are too flat to really live up to their billing.
Berendt's book is technically non-fiction, and split into two parts: the first half recounts the local characters he meets and the second is an in depth account of the four murder trials of Jim Williams, a local millionaire and art dealer who is accused of shooting and killing his 21-year old male lover. The first part is too much Discovery Channel, the second too much Court TV.
In the end the book is caught between being fiction and being non-fiction. We know that Berendt has invented events from whole cloth; though he writes himself as a significant figure in Williams' life from before the shooting, we know that he didn't arrive in Savannah until after the trial had begun. By nature the plot must be fictionalized to the point of no longer being non-fiction, but the writing is so dry and lacking in the "atmosphere" of Savannah that it makes for poor fiction.
Just see the movie; it has John Cusack.