The tone of The Borrowers is as proper as afternoon tea, as playful as a little cocker spaniel puppy, and as sympathetic as your best friend. Who wouldn't want to read a book with a tone like that?
Perhaps owing to the time period in which the book was written (the 1950s) and the location (England) the tone of the novel is exceedingly proper. Characters say things like, "really […] he is a very angry sort of boy" (9.51) when really they might just have said, "Jeez, what a jerk."
Throughout the entire novel, the characters' thoughts, words, and actions of youths and adults alike are filtered through a sieve of Victorian appropriateness, so only the proper stuff remains. Don't you get a sense of stepping into a different time when you pick up the novel? Perhaps the tone is part of that.
Even though the tone is proper, it's never stuck-up or stiff. In fact, it's downright playful at times, making fun of some of the characters with a healthy dose of whimsy, like in this description of Homily:
She curled her hair nearly every evening nowadays and, since the house was more or less straight, she would occasionally change for dinner into a satin dress; it hung like a sack, but Homily called it "Grecian." (16.8)
Even though Homily's dress looks ridiculous ("it hung like a sack"), the narrator playfully says that Homily called it "Grecian," as if it was high-fashion in the style of the Greeks. The tone gently pokes fun at her vanity and desire for high society, but never in a mean-spirited way. Homily's not the worst person in the world, she's just got some less-than awesome values.
But before you go thinking that this book is just a snooty, teasing tome, consider this: we readers feel a ton of sympathy for our main characters, like Arrietty and her family. Hey, we've even got good vibes for the more minor characters, like Great-Aunt Sophy, too.
Think about it. Great-Aunt Sophy is more than a little off her rocker. But instead of poking fun of the old lady for thinking that Arrietty's parents come out of the wine decanter, we get this rather sympathetic take:
"She thinks my father comes out of the decanter," said Arrietty, "and one day when I'm older he's going to take me there and She'll think I've come up out of the decanter too […] once he took my mother, and She perked up like anything and kept asking after her and why didn't she come anymore and saying they watered the Madeira because once, She says, She saw a little man and a little woman and now she only sees a little man…" (10.1)
Arrietty is totally nonjudgmental here. She just tells it like it is, and the resulting picture we get of Great-Aunt Sophy is sweet, cute, and oh-so endearing. This tone helps build sympathy for the borrower-friendly humans of the book. We know Great-Aunt Sophy is a good gal because she gets a kick out of the borrowers rather than, say, trying to exterminate them with rat poison.