This is a novel based on Amano-sensei's ARIA manga. As with just about all the spin-off novels, it has a totally new story, and is written by someone other than the original creator.
Story is about an opera, "Mizu no Yousei," lost when the original Venezia was consumed by the rising ocean. Obviously, this is a fiction, although one never knows if such a thing would really happen at some point. Anyway, due to this, Valore Playhouse in Venezia was not able to start "Mizu no Yousei" performances. All the involved people scattered and the original materials (scripts, music scores, pamphlets, etc.) were lost as well. Valore Playhouse was recreated in Neo Venezia at some point. however.
New owner, Angela, of the now abandoned Valore Playhouse decides to revive the "Mizu no Yousei" by making up the lost parts. But her brother, Asch, is not happy with the idea of faked version. Asch comes to Neo Venezia looking for the original materials and meets Akari, Alice, and Aika who were in the playhouse. Technically, Akari, Alice, and Aika broke into the place hoping to see the rumored ghost. Either way, everyone gets involved in the search for the original materials.
By the way, they do meet the ghost, Adelina, who was supposed to be the leading actress in the never-performed opera in Venezia.
Technically, the book is really well written. By that, I mean that it makes use of a lot of materials from the original manga. If you've read the manga, you'll find many familiar concepts used in the novel. For the people who are not familiar with the original manga, the book briefly explains most of these concepts. To me, these explanations disturb the story flow. Personally, I can't imagine there being all that many readers who did had not read the original manga, so I wish the author would have done away with the explanations. Even the anime did away with a lot of these introductions.
Other than that, it's interesting enough. Somehow, though, it didn't leave me with a strong impression of any kind. If you're a "must have everything ARIA" type fan, you'll probably find this book interesting enough. Otherwise, I'd personally recommend you stick to the manga, and maybe some of the drama CDs.
