How does Christopher find his mothers letters?

 Christopher finds his mother's letters in the novel "The Curious Incident of the Dog in the Night-Time" by Mark Haddon. Here's how the event unfolds step by step:

1. Christopher Boone, the protagonist, is rummaging through his father’s bedroom for something unrelated to the letters – he is actually searching for his book which his father had taken from him.

2. While searching, he comes across a shirt box that is hidden in a cupboard, which is tucked away with other objects.

3. Curious about the contents of the box, Christopher opens it and discovers a stack of letters addressed to him from his mother.

4. Christopher is shocked and confused, as his father had told him that his mother had died of a heart attack. The letters reveal that this was a lie and that his mother is in fact alive and living in London.

5. The discovery of these letters is extremely significant as it not only uncovers the lie told by his father, but it also sets off a series of events that lead Christopher to seek out his mother.