Many Young Adult book series have short stories (#0.5, #1.5 etc) or novellas.
Here are a few that I want to read or have read.
Free Four (Divergent) - Destroy Me (Shatter Me)
Sword of Hades (Percy Jackson) - Radiant (Unearthly)
Delirium Stories: Hana, Annabel, Raven and Alex (Delirium) - The Bane Chronicles (The Mortal Instruments)
Iron Legends (Iron Fey) - Harry Potter: The Prequel (Harry Potter)
Midnight Sun (Twilight) - The Tales of Dunk and Egg (A Song of Ice and Fire)
Tell me your favourite short story!