JOHNNY AND THE DEAD By Terry Pratchett (Doubleday, $170) MOST adolescents like to sneak a peek at books written for adults. Ever since Terry Pratchett came along, adults have been furtively running off with books written for children. Mind you, many of his books are written for inbetweenies, or for the young person who permanently lives in all of us whatever our age. Particularly good in that category is Only You Can Save Mankind, the story of video games but told partly from the point of view of the aliens who are forever being blown to pieces on the screen. It tells the tale of how one schoolboy crosses the line and drives his spaceship into the universe on the other side of the screen. The new book,Johnny and the Dead, is also a winner. It features the same schoolboy, but stands alone as a novel. A faceless company wants to build an office block on an old council cemetery. The ghosts living there become upset, rise from their graves and enlist 12-year-old Johnny Maxwell to help them fight for their site. This gives rise to much riotous humour. ''Tell them we are not going to take this lying down!'' shouts one of the dead people. It's not easy for Johnny to cope with the idea of death. ''The only dead people he had known had been Mr Page, who'd died of something, and his great-grandmother, who'd been 96 and who had just generally died.'' One of the delights of Mr Pratchett's works is the way he picks up the earnest absurdities of youthful conversation. One of Johnny's friends reports that an adult has said that witches are abroad on Halloween. ''What? Like . . . Majorca and places?'' asks Johnny. ''Suppose so.'' Despite the fantasy elements of the novels, the world in which the characters live is gritty and real. The sky is not blue, like in TV commercials. ''Most of the time the sky was that odd, soapy colour you'd get if you lived in a Tupperware box.'' Some of the gang live in a low-class housing block. ''The lifts hadn't worked properly since 1966. They lurked in the basement, too scared to go anywhere else,'' says the writer. Johnny Maxwell makes an appealing central character because he is never heroic, but lives in a perpetual state of nervousness. And so would you if you lived in a rough neighbourhood which includes Clint the dog, ''which had reputedly been banned from theRottweiler-Pit Bull Terrier Crossbreed Club for being too nasty''. Hint: buy Terry Pratchett's books as presents for your younger friends - but pick them up good and early, because you may just feel like reading them yourself first.