The brilliance of Lauren Child is the result of a unique chemical reaction between a glorious sense of style and a smartly original take on real life. The mass audience her books have attracted shows that off-beat humour is in no way incompatible with universal appeal.

Ruby Redfort: Look into my Eyes is a great, if obvious, idea. Take a sassy, super-clever, code-cracking character from the Clarice Bean books and put her in her own full-length novel. Play up the bright and shiny retro-stylish America in which she lives (and the bright and shiny American wisecracks she likes so much), team her with a secret spy agency called Spectrum, hand her an array of ingenious gadgets for whisking her out of life-threatening situations, pit her against the world's most villainous villains, and throw at her a range of devilish problems including a missing code, the theft of her housekeeper, Mrs Digby, and the impending heist of a shipment of gold bullion en route from Switzerland to the safest safe in America, which happens to be in Ruby's hometown. It's a tremendous idea  for an animated movie. For a long, pictureless novel, which readers will need to bring to life in their own imaginations, it may be a strategic error.

The detail is charming but corny. There are 79 Ruby Rules including "Panic Will Freeze Your Brain" and "What To Do When You Meet a Bear  Wish You Hadn't!", a toaster that's also a communicating device ("Is that piece of toast private or can anyone eat it?") and an amusing brace of airhead parents amiably negligent of their daughter. There's also a terrific mystery man called Hitch, the smoothest operator since Jeeves and very endearing to boot.

But there's an awful lot of detail and it tends to pile up all over the place, blurring the characters, clogging the dialogue and cluttering the plot. The theft-of-Mrs-Digby subplot appears in brief flashes at set intervals, breaking in like commercials for another story entirely. In the main story, clues arise at suspiciously convenient moments, like brightly coloured balloons, to be promptly solved by Ruby with a knowing wisecrack.

Codes and puzzles are at the heart of it all, some very nifty indeed, some a little shopworn, and others rather lame. Ruby herself is an odd mixture of likeable sauciness and child-genius stereotype. A child-genius is a challenging thing for an author to create, and I'm not convinced. There are some great moments  Ruby's exchanges with the Spectrum agents are funny and warm  but too often I'm told how clever she is (she's reading War and Peace in the original Russian, apparently) without seeing her intelligence in action for myself. Worse, I don't feel I get to know her. The chemistry with her best friend Clancy is intermittent, and she struggles to express herself beyond jokes and the endless "Jeepers", "Darn it" and "Boy, is this guy a prize potato head".

She's a cartoon who lives in a cartoon world, and I fear the brilliant premise, charming detail and occasional wonderful moments can't sustain her through the long haul of a novel.

 Simon Mason's Moon Pie is published by David Fickling.