The ending of Changes was, in a word, epic. It also ended in a manner which left fans sincerely curious as to how the series would continue. If you haven't read up to Changes, now would be a good time to stop as it's impossible to discuss Ghost Story without discussing the finale of the previous book.
Harry dies at the end of Changes.
Shot by a sniper rifle, probably wielded by Kincaid the Assassin (a rival/friend/enemy), Harry fell into Lake Michigan and disappeared. Much like Superman, you know Harry was going to come back but some believed the series might have a major time skip followed by a switching to Harry's newly discovered daughter.
Instead, the story starts with the very intriguing premise of Harry as a ghost. This status, again, is something no serious fan would expect to last but allows the consequences from the previous book to continue. Awakening in a state between life and death, Harry is charged by the Archangel Uriel to investigate the circumstances of his death and becomes embroiled in a conflict with the ghost of an enemy.
The book's appeal is watching how Harry's friends and associates have dealt with a world without their protector. It is a sobering realization for Harry that Chicago is a far darker place without him yet his friends are capable of defending it. They have formed a "Justice League of America" which works as a network across America, fighting against the aggressively expansionist monsters of the world.
The destruction of the Red Court of Vampires saved the White Council but like the fall of many dictators, the sudden chaos and power vacuum hasn't made things easier for humanity. The Formor Court is a collection of gods, fairies, and monsters kept in check by the Red Court and White Council's power for centuries but now one is gone while the other is terribly weakened.
Perfect for expansionism.
Throughout the book, Harry struggles to deal with his friends who are used to dealing with imposters and not wanting to believe their friend is dead. I loved Murphy's reaction best because she has stepped up to be the new hero of Chicago but the "unsolved" mystery of Harry's murder burdens her tremendously. Seeing her come to terms with Harry's death in the way a fan might is a great bit of writing.
Also great is the development of Molly Carpenter. Previously Harry's spunky Goth girl sidekick, she's evolved into the "Ragged Lady." A woman who uses illusions and misdirection to punish not only the supernatural evils of Chicago but its corrupt police force. The change was unexpected, heartbreaking, and yet believable given what we know about her.
The action in the book is well-written with a conflict between Harry and an archwizard's ghost. The fact Harry, as a ghost, is deprived of the majority of his supernatural mojo makes the story have higher stakes than normal. You can usually count on Harry to be able to blast himself out of problems and he doesn't have that as an option here.
The Corpsetaker's ghost and her spirit army reminds me of the old Wraith: The Oblivion tabletop roleplaying game, which isn't a bad thing. I also enjoyed reading about how Harry struggles to adjust to being an invisible incorporal specter who can't do any magic. The Formor are worse than the Red Court in many ways and I look forward to seeing them as foes in future volumes. I can't say I really buy the world is worse off than when the Red Court was alive, though, since they were such a titanic force for evil. You'd think at least some of the world would be a better place without soulless marauding psychopaths.
In conclusion, Ghost Story is a great continuation of the saga and a way to write Harry out of the corner he was pushed into. The new status quo is exciting and full of promise as is the possibilities opened up by Harry's status at the end.