Diary Entries, Intimate, Casual
If you've read the "Tone" section already, then you know this book is epistolary. Specifically, it's written as a series of diary entries penned by one Georgia Nicolson, and since she's only fourteen and only writing for her own delight and documentation, things stay pretty personal and pretty casual on these pages.
Because this is her diary, Georgia lets it all hang out, be it about her own cleanliness habits—"Uh-oh, we are going up the stairs to Peter's room. He hasn't said a word. His room is much tidier than mine" (3.347)—or her feelings about her baby sister: "She's so nice, although a bit smelly" (1.59). There's nothing fancy or formal about the language because she's a teen writing for herself, not, say, a grad student writing their dissertation. This keeps the book relatable for teens, which is perfect since they're pretty much the only people Georgia's interested in anyway.