Sarah Lucas There actually is a fourth book, sort of. In the series Darkness Rising it talks about more teens in a similar situation. In the last book the characters of both books unite to fight against the bad guys (forget their names)
Sara Apparently there is a novella and also a short story that happens after The Reckoning. They are Belonging and Facing Facts. This is what it says about Belonging, "(the novella set after The Reckoning), Derek's werewolf family comes to claim him. Narrated by Derek." This is what it says about Facing Facts, "(short story set after The Reckoning), Tori discovers who her father is, and Chloe deals with a vengeful ghost. Narrated by Chloe." There are also two other novellas that show what happened to Derek and Simon after they get separated from Chloe at the warehouse and Simon and Tori after Derek and Chloe get off the bus. And as several other people have mentioned, there is also the Darkness Rising trilogy in which they do make an appearance.