The Goldfish boy by Lisa Thompson

This empathetic debut is a middle-grade whodunit 

with a very special heart. Kirkus

It might sound strange but I miss the brother I never met.  The one who died because of me.

Matthew is trapped.  He is trapped in his room.  He is trapped in his fear of germs.  He is totally freaked out by the number 13. He is trapped in his guilt.  Matthew is suffering. He constantly needs to wash his hands.  He cannot touch any surface because he is so afraid of becoming ill and making other ill.  Matthew spends his long days looking out the window and making detailed notes about life in his cul-de-sac street. 

The man next door is also quite obsessive. Mr Charles cares for his roses with great precision.  When his daughter drops his two grand children off for an extended visit his world is turned upside down.  Matthew is the only person who sees the truth about Casey the six year old.  Teddy throws her doll in to the fish pond and she retaliates :

"Stretching her arms as if she were about to do a conjuring trick, the little girl ran at her brother. Her hands hit him with such force his little head jolted back, then he toppled forwards, straight into the pond."

Sadly for everyone involved no one, not Matthew, not the police, not Mr Charles, link this event with the disappearance of Teddy aged 15 months. Meanwhile Matthew continues to suffer. He has not been able to share his guilt about the death of his brother five years ago.  The counselor tries to help and she does make some progress towards the end of the story but the real healing is provided by Melody - a girl who also lives in his street.  They need to band together to solve the mystery of Teddy and while they do this their friendship is forged. Everything is not solved at the end but it is reassuring to know Matthew can recover.

Here is a review with more plot details.  I would follow Goldfish boy with Counting by 7s, The curious incident of the dog in the night time, My life as an alphabet and The naming of Tishkin Silk.

Here is an alternate cover which I actually prefer.  I think it matches the complex themes of the book and may appeal to an older reader.