"Mary Anne's Bad-Luck Mystery (The Baby-Sitters Club #17)" by Ann M. Martin

Mary Anne should never have thrown away that chain letter she got in the mail. Ever since she did, bad things have been happening...to everybody in the Baby-Sitters Club. With Halloween coming up, Mary Anne's even more worried-what kind of spooky thing will happen next?

Then Mary Anne finds a new note in her mailbox: Wear this bad-luck charm, it says. OR ELSE. Mary Anne's got to do what the note says. But who sent the charm? And why did this person send it to Mary Anne?

If the Baby-Sitters don't solve this mystery soon, their bad luck might never stop!