A man goes to the library to find the best book about committing suicide. So when he asks the librarian, "What's the best book on committing suicide?" The librarian said, "Oh, fuck off...you won't bring it back anyway."
Library Book on Suicide Request Gone Wrong
Explanation
The joke plays on the dark humor of suicide, mixing it with the annoyance of someone not returning a library book. It's unexpected for the librarian to react that way, making it funny.