From my childhood (pre 12) three books really stayed with me:
Woof by Alan Ahlberg - about a boy who turns into a dog three times. I loved it then (and now) because it doesn't resolve. It concludes that sometimes strange things happen and you never know why, but you can have fun theorising.
The Machine Gunners by Robert Westall - about a group of kids in WW2 who find a downed german plane and steal the machine gun from it.
The Turbulent Term of Tyke Tyler by Gene Kemp - an astonishing portrayal of the friendship between a tough but troubled kid and another kid with learning difficulties/special needs. Also an interesting early kids book about gender roles. Read it again recently, it's still amazing and I'm surprised it isn't better known.
I'm pretty sure you could trace a lot of my life philosophy and tastes in books, tv and films, back to these three books.
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It's dark. Is it always this dark?