man with memory loss and personality types

[Simon]

I read a book about a man with memory loss

after some investigation he found it was due to a machine that was able to change his personality type
there was also a murder/death in the days of testing this machine

at the end of the book, this machine is used to potentially change everyone's personality type (there were 3 types)

anyone know the title?

[Kris]

Simon, around what year did you read this book?

What's the genre (which we need to add to the topic header) - e.g., adult fiction or nonfiction? science fiction, fantasy, thriller, mystery, etc.?

What is the man's occupation?
Story's location/country and time period?