The next day, Christopher goes to school and tells Siobhan that he won't be able to finish the book, because his dad made him promise to stop detecting.
She's okay with that – the book is really good as is, and not all books have nice tidy conclusions to their mysteries anyway (perhaps that's what makes them mysterious).
They have a brief conversation about Mr. and Mrs. Shears, but Siobhan can't offer Christopher any help because she doesn't know the couple.
Each of the next two days, Christopher sees four yellow cars in a row on the way to school. You know what that means: Black Days.