People like to read books about serial killers. Why do farmers grow bananas? Answer: people like bananas.
First, it's a fascinating subject. Second, fascinating subjects sell books.
Authors write books.
authors write books
Writers write about what they like.
James Patterson writes books with other authors because he is not smart enough to write his own book.
Erin hunter is four authors and two of the authors never met so they don't write the books in a specific place
Authors are the people who write books. EG: JK.Rowling is the Author of harry potter
Because they sell.
Many authors collaborate and write books together.
Serial Killers Psychology is a complex thing to analyse. I will not write such a long answer to a question but instead, in the related links box below, I will post a website where you could start your search.
Serial means a series of crimes, usually one at a time, with a 'cooling off' period in between kills. The cooling off time depends on the individual killer. Jack The Ripper killed two women in the space of an hour. Some killers may not kill again for several years.
It depends on how many books they write a year and how popular the books are.