But honestly, the answer is always "the book I'm currently writing," because that's the only time when the book is just "mine." That's when no one else has read it yet, and no one knows what's going to happen. It's my secret time with the characters, and it's a very magical thing.
Not that it's not magical after it gets published--it definitely is! But it's also different once it's out there for readers to read. Then it's YOURS, not mine, since books belong to the readers. But drafts? Drafts belong to the writers. :)