Side books and short form books do not count towards "Complete a Book" Challenges. These include:
The Freshman: Love Bites
The Freshman: Snowed In
The Freshman: Game of Love
Chris: Luxury Getaway
James: Masquerade
Kaitlyn: Perfect Match
Hartfeld Horror Story
The Elementalists: Winters Past
The Royal Romance: The Royal Holiday
Bloodbound: Dark Solstice
Bloodbound: Origins 
Desire & Decorum: First Winter
The Dalton Affair
The Deadliest Gambit
Blades TTRPG Companion
Villainous Valentine
Make That Date
Crimes of Passion: The Proposal
Open Hearts on Fire