Well, so you read til where he finds his mother's letters.
So he now knows that his mother isn't dead.
I think he faints, or rather, vomits & falls asleep over the letters, coz he's in total shock. His father finds him & the letters, and tries to explain.
I don't know if it was at this point that he finds out about the dog.
Think that was a little while later.
Well, it turns out that his mother had run off with their neighbour.
Which had obviously been very upsetting for her father.
And very upsetting for the neighbour's wife - the owner of the dead dog.
It turns out that his dad & the female neighbour ended up comforting each other... ie they had a relationship. In the end, the neighbour couldn't take it any more (this was of course due to the stress with the autistic protagonist).
Well, at some point in the story, when the father is insistent again that his son stop trying to solve the mystery (as in, who killed the dog?), and eventually admits that the reason for his reaction is that he himself had killed the dog! Coz he was angry with his neighbour, who he had had the relationship with...
Of course the protagonist (what was his name??? Aidan?? oder so ähnlich? can't remember!), well, he can't understand the difference between killing an animal and killing a person, so he's totally horrified & thinks his dad might kill him. He hides in or behind the shed in the back garden, his father doesn't find him, and when his father drives away in the car, I think he runs back inside, grabs a few things, and runs away, he wants to visit his mum.
At the train station he sits down on a bench, and falls into some sort of trance. He 'wakes up' when someone, a woman working at a newsagents there, I thinks, speaks to him. At this stage the whole day has passed without him being aware of anything around him.
I can't remember exactly now, but I think in the end there are security guards or policemen trying to catch him, coz they know his father is looking for him (... but he's scared to death of his father!).
I think that happens on the train. And I think he hides in a luggage compartment among suitcases, if I'm not mistaken.
Anyway, he gets to the town his mother lives in, I have no recollection of how he gets to her house. But he is eventually found outside.
His mother is of course very sorry for deserting her son, and absolutely shocked and angered that her ex husband had told him she was dead!
She says that of course he can stay with her, but it doesn't take long until her partner (the ex neighbour) starts throwing fits.
Then, of course, the dad arrives. He's of course still dead scared of him, and won't let him come near him. Somehow they manange to talk, through the door or whatever.
Don't know exactly what happens then - at least the mother decides that her son is her first priority, and dumps her partner, who wants her all to himself.
She goes back 'home' with father and son, the couple want to give their relationship another chance.
But he really needs his mother now - the trust in his father is completely destroyed, he still won't let him near him, but he manages to live under the same roof as him.
The book ends with him at the very start of a long process of building up some trust again. His dad gives him a dog as a present, which I think helps quite a bit.
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Anyway, so much to the 'story' itself. Seeing as you already know the book itself, I won't have to explain that this doesn't in any way capture the amazing way in which the author manages to capture the emotions and the way of thinking of an autistic teenager, written in the first person. It's just amazing, and it certainly belongs to my all-time favourites.
All the best,
Isabel