Even though there's a third person narrator, we get most of the story from Coraline's point of view, hearing her thoughts and her feelings. This means that Coraline herself pretty much determines the tone of the novel. All the tones we get come largely from Coraline's own emotions: when she's scared, or relieved, or bored, or excited, we hear it.
Something that we feel from Coraline (and so, the narrator) throughout the story is a sense of curiosity. Coraline wants to know about everything:
She said to her mother, "Where does that door go?"
"It has to go somewhere." (1.47-49)
The narrator lets us hear Coraline's curiosity, which makes us curious, too.
In addition to her curiosity, Coraline is also very emotional. Yeah, we know, this is a really broad word, but it really sums up the tone of the book. Coraline is a very emotional person, which means that she feels lots of things; and the narrator never really hides what Coraline is feeling. When she's upset, we hear it in the tone. When she's happy, we hear that, too. As you can imagine, that gives us some major ups and downs throughout the story.
Because of this roller coaster of emotions, the narrator makes sure to calm things down. He's able to do this only because – as opposed to Coraline – he's not in the moment: he's just recounting it. The scene where Coraline is about to return to the other world demonstrates this pretty well:
There was no brick wall on the other side of the door; only darkness. A cold wind blew through the passageway.
Coraline made no move to walk through the door. (5.73-74)
Coraline herself probably isn't feeling nearly as calm as the narrator describes here. The narrator just tells us what's happening: he lets us know that Coraline is scared without making the story sound scary. The narrator takes over the tone here and keeps things calm in order to tell us what's happening. And maybe to calm us down a bit, too.