The Discworld Series
Terry Pratchett's Discworld books have long delighted readers from 10 to 90+ and are very popular with many boys who scorn other fiction. Now he's written one aimed particularly at children, although adults will enjoy it too.
Maurice and his Educated Rodents
Take a bunch of intelligent rats and a boy with a pipe. Add a streetwise talking cat and you've got a great pied piper scam. All goes well until the team arrive in Bad Blintz, a town that already has a bigger rat problem than it can handle, and they meet Malicia, a girl who has trouble telling fact from fiction. This is Terry Pratchett at his best with skillfully drawn characters whose interaction provides most of the laughs and a strong plot which keeps you turning the pages. You don't need to have read any of the other Discworld novels before but fans will be delighted to see the Death of Rats in a brief, squeaking part
For confident readers from 8 to 108
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Those who enjoy Maurice are likely to want to move onto the main Discworld series. Set on a flat world carried through space on the back of a giant turtle, they are packed with humour and an amazing range of characters, many of whom appear in book after book. These recurring characters and their associated running jokes mean you can easily miss some of the humour if you start with one of the later books in the series. (You may even miss some of the plot if you don't realise that Death speaks in capital letters.) On the other hand, don't fall into the trap of thinking you should start at the beginning as the first book hasn't such a strong plot as the later ones.
Most Discworld fans I have spoken to recommend starting with Mort - the story of a young man who takes a job as Death's apprentice and gets into trouble when he tries to change destiny.
Other titles worth considering as starting points for young readers are
Wyrd Sisters, the first of the books featuring Pratchett's wonderful trio of witches
(warning - when drunk, Nanny Ogg sings a little ditty called "a hedgehog can never be buggered at all" although she never gets past the first couple of lines)
Guards, Guards - the first of the books about Ankh Morpork's City Watch.
Try reading the books yourself too - then you can share the jokes together.