Conversational and Descriptive
Most of the time, this book reads like Ellie is sitting in front of us, talking away, telling us the story in a regular teenager way. She doesn't try to make it sound formal, just natural and believable. Check out this quote from the beginning of the book:
So that was the Famous Five. I guess Corrie and I made it the Secret Seven. Hah! Those books don't have a lot of bearing on what's happened to us. (1.102)
Phrasing like "So" and "I guess" and "Hah" are a big part of what make this a conversational writing style. It's how people actually talk, you know? This matters because if the book were written differently—say it was super formal—we wouldn't believe a teenager was the one doing the storytelling. The writing style fits the framing of the text as a story being written by an ordinary teen.
Just because she's a teen doesn't mean the storytelling is flat or lacking description, though—Ellie might be young, but she's not afraid to dive into description, helping readers really see this terrifying situation in all its glory. So when Ellie describes what she sees while on sentry duty, readers get the whole scene, too:
We sat there, looking out across the paddocks to the dark fragment of road in the distance, lying across the countryside like a thin black snake. (9.18)
Can't you just picture this? It's as though we're sitting right next to Ellie as she keeps watch—words like "dark" and "fragment" and "snake" imbue the scene with ominous undertones, helping us both understand what Ellie literally sees and how she feels about the landscape before her given their difficult circumstances. Not too shabby for a teenager, if we do say so ourselves.