Enid Blyton 
The Mystery of Banshee Towers [EPUB ebook] 

I do wish old Fatty would buck up and come back from wherever he’s staying, ” said Bets. “We’ve had almost a week of the holidays without him already—such a waste!”
“He’s coming back today, ” said Pip, passing a post-card across the breakfast-table to his young sister. “Here’s a card from him. Three cheers!”
Bets read the card out loud. “Back tomorrow by bus from Warling. Meet me at bus stop if you can. What about a nice juicy mystery? I feel just about ready for one. Fatty.”
“A nice juicy what?” said her mother, puzzled.
“Mystery, ” said Bets, her eyes shining. “You know how something always seems to happen when Fatty’s about, Mother—there was the mystery of the Pantomime cat—and the mystery of the Vanished Prince—and . . .”
Her father groaned. “Look, Bets—I’m tired of all these adventures and strange happenings that seem to pop up whenever your friend Frederick is about. Just try and steer clear of any trouble these holidays. I was hoping that Frederick was staying away for a nice long time.”
“I wish you wouldn’t call him Frederick, Daddy, ” said Bets. “It does sound so silly.”
“I should have thought that Frederick was a much better name for a boy in his teens, than the absurd name of Fatty, ” said her father. “I wonder Frederick allows people to call him by that old nickname now.”
“But Fatty is fat, and the name suits him, ” said Pip. “Anyway I don’t think my nickname is very suitable for me now that I’m a bit older. Why can’t I be called by my proper name of Philip, instead of Pip?”