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King's Death Garden by Ann Halam, a pseudonym of Arthur C. Clarke & Philip K. Dick Award-winner Gwyneth Jones (
ISFDB,
Wikipedia), 1st in her Ghosts and Other Hauntings series tween/YA urban sf/fantasy/horror tales originally out from Orchard Books/Penguin in 1986. Some of Jones' other "Ann Halam" fantasies were still on the school library bookshelf when I was a kid and directly contributed to the formation of my lifelong speculative fiction reading habit.
Maurice's family has left for the Middle East: Maurice: asthmatic, plagued by allergies, self-centred, impossible semi-invalid, is sent to live with his great aunt in Brighton, and become fascinated by the beautiful old cemetery next door. Isolated from the real world around him, he's playing a dangerous game with the "strange scientific phenomena" he encounters in there, but despite some alarming warnings, hinting at where he's heading, he just can't stay away.
Nimrod by "Ann Halam", 6th in the series and originally out from Dolphin/Orion Books in 1999 under the title
The N.I.M.R.O.D. Conspiracy.
Stacey vanished three years ago. Alan knows she's dead: he knows what happened because it was all his fault. But his mother refuses to believe it. Mum's endless, hopeless search for her baby, her lost little daughter, is the worst misery of Alan's life-after-Stacey. Going nowhere, filling shelves; seeing his dead little sister' ghost. Then Mum discovers NIMROD, the psychic missing persons agency. These people claim they can pluck messages from lost souls (living or dead) from the aether, and convert them into text on a computer screen. They're just another bunch of callous chancers, as far as Alan's concerned. But Mum's convinced, so Alan gets drawn in.
The search for Stacy becomes Alan's search for the truth about NIMROD, and leads him into a bewildering underworld of James Bond gadgets, luxury yachts, conspiracy, blackmail; secret safe houses in the backstreets of his seaside town; and even aliens from outer space. Who are these people? They've got to be faking their "Stacey" messages, but how can they know what they know? And if they're crooks, why don't they go after someone with money? What can they have to gain from targetting Alan and his Mum?