He swam the Channel before he wrote any of them
In July 2006 WALLIAMS swam from England to France in ten and a half hours, at the time one of the faster crossings by a Briton. In 2011 he swam 140 miles of the Thames over eight days and became seriously ill from the water. Both raised millions for charity, and neither had anything to do with books — the first novel came out in 2008, between the two swims.
He had spent the preceding years as half of a sketch comedy double act, writing and performing characters for television. That is an unusual route into children's fiction and it shows in the prose, which is built almost entirely out of jokes, catchphrases and people being ridiculous at each other.
The illustrations do half the work
Quentin Blake illustrated the first two books, which is a direct line from Roald Dahl, and Tony Ross took over afterwards. The pictures are not decoration: they break up the page, carry a lot of the comedy, and make a 300-page book feel manageable to a child who has not read one that long before.
This is the most underrated thing about the modern children's bestseller. A reader moving up from picture books faces a wall of solid text, and the illustrated novel is the ramp. Publishers know exactly what they are doing with the white space, the short chapters and the wobbly hand-drawn headings, and none of it is accidental.
Under the silliness they are nearly all about loneliness
Look at what the books are actually about. GANGSTA GRANNY is a boy who finds his weekly visits to his grandmother unbearably dull, and the story is about discovering she had a whole life before him. The MIDNIGHT GANG is set on a children's hospital ward. Mr STINK is about homelessness. The Boy in the Dress is about a boy who likes wearing one and the friends who work out they do not mind.
The comedy is the delivery mechanism. A book that announced it was about grief, or an elderly relative you have not bothered with, would not get picked up in a school library; a book with a DEMON DENTIST or a RATBURGER on the cover does, and the other thing arrives while the reader is laughing.
Everything in the grid
The titles: GANGSTA GRANNY, DEMON DENTIST, RATBURGER, Mr STINK, the MIDNIGHT GANG, SPACEBOY, AWFUL AUNTIE and the various MEGA MONSTER, WORST CHILDREN and BEAST collections that followed.
Then the tone words the covers keep promising — FUNNY, NAUGHTY, SLIME, ESCAPE, COMEDY. The comparison to Dahl is made constantly, usually by publishers, and it is fair in one specific way: both understood that a child will follow a story almost anywhere as long as an adult in it is being thoroughly, satisfyingly awful. Where they differ is the ending — Dahl tends to let the villain be destroyed, while these books more often let somebody turn out to have been misunderstood, which is a gentler shape and a slightly less memorable one.