Grundle’s Hope - Occurs during Book 2, but contains spoilers for Books 3 & 4.
Age Before Duty - Occurs during Book 1, but focuses on Elliah (a character from Book2 -4), while Red is unraveling the mysterious attacks on her building.
Losing Faith, a Calyx Twins Story - Takes place in the time frame of Book 3, with characters met during Book 3.
I collected them all together and wrapped it up in a sequence called “The Guardian League: Urban Legends”. (pdf) (epub)