Seeking the name of a mystery novel and the name of the author

The content was about several students in the same dormitory being killed one after another. In the end, it was discovered that the murderer was from the same dormitory. The murderer was a child from the countryside. He had an improper relationship with the dormitory manager and put a letter in a book. Then, he killed the people in the order he borrowed the books.
After reading it, he had forgotten the name. Please!
Later, the author wrote another book, which was a continuation of the first book. The book started off with a story about a murderer in a city who killed many people and put the captain of the city's criminal police in a difficult position. Later on, someone introduced him to a student, a boy who was very good at reasoning and analysis. This boy was the one in the first book who deduced who the murderer was among the surviving children in the same dormitory.
Who knew? The author was said to be a big fat man.

Answer 1

The name of this mystery novel was " White Night Walk ", and the author was Keigo Higashino.