There are 43 letters in total - all addressed to Christopher.
In one letter his mother writes about happy times when they bought Christopher a train set and he loved it. In the second letter his mother explains in some detail why she left him. She says she was hot tempered and not as good as his father at looking after him. She fell in love with Eileen's Roger because she was lonely and he was lonely, and when he asked her to move to London with him she agreed, because she thought it would be better for everyone. In the third she writes that she hopes he got the present she sent.
After the forth letter Christopher feels giddy and sick that his father lied to him that his mother was dead. He lies in bed curled up. Christopher doesn't remember much after that - just waking up after dark with vomit on his bed.
This section is the turning point of the novel. It is through reading that Christopher uncovers the biggest secret of all: Haddon really celebrates the power of communication through the written word. In the way that Christopher has enlightened us about how he sees the world by writing his story, his mother’s letters shed light on the mystery that has been puzzling him. Note that Christopher's spelling is great but his mum's isn't!