Rare Beasts, a children's book by Charles Ogden, concerns Edgar and Ellen, twins who live more or less by themselves in a big house out on the edge of town. The twins have rather nasty dispositions and the only thing they enjoy more than bedeviling each other is playing cruel pranks on the other children and residents in the town.
In this particular book, they steal all the town's pets and decorate them with paint, glitter, sticks, etc. to look like undiscovered rare animals that they hope to sell for great profit to collectors. However, none of their potential marks, er, customers work out, and in the end their plan is disclosed publicly and the twins get their comeuppance.
I liked the amusingly cynical narration and humorous little details that are really for parents and may go over a lot of reader's heads (I'm assuming most readers of this are probably 8-10). For instance, the absence of the twins' parents is explained by an off-handed remark that "they had long since departed on an extended 'around the world' holiday. At least that's what it said in the note they left behind." It reminds me of Lemony Snicket's Series of Unfortunate Events books, which I suppose was probably an inspiration.
My son hasn't read it yet so I can't get his opinion, but I think he'll probably like it. It's got a kind of morbid but immature humor he'll enjoy and is scary, but not too much.