The Solstice War is a conflict that spans the world of Aer, not just the Ayvartan continent. These stories might be considered “Extras” but they follow characters whose lives briefly brush with the intractable force of War, and who in their own way have an impact.
You should read at least “The World Ablaze” and The Councils Divided in the main story before reading these side-stories. Events in these stories happen contemporaneously to the “Books” — timestamp dates in the Aerean calendar are used like in the main story.
Salva’s Taboo Exchanges
Extra: Salva’s Taboo Exchanges, Part One (Salvatrice)
Extra: Salva’s Taboo Exchanges, Part Two (Salvatrice)
Extra: Salva’s Taboo Exchanges, Part Three (Salvatrice)
Extra: Salva’s Taboo Exchanges, Part Four (Salvatrice)
Extra: Salva’s Taboo Exchanges, Part Five (Byanca)
Extra: Salva’s Taboo Exchanges, Part Six (Salvatrice)
Extra: Salva’s Taboo Exchanges, Part Seven (Salvatrice)
Extra: Salva’s Taboo Exchanges, Part Eight (Salvatrice)
Miscellaneous
Extra: The Gloom’s Bad News (Zhukova)
Extra: The Exiles, Part One (Bercik)
Extra: The Exiles, Part Two (Bercik)
Extra: From The Solstice Archive, Part One (Daksha Kansal)
Extra: THE JUSTICE OF JOHANNES JAGER, Part One (Karla Schicksal)
Extra: THE JUSTICE OF JOHANNES JAGER, Part Two (Karla Schicksal)
Extra: The Covered Box (SUPER HEADCANON)
Extra: Matters of Imagination (SUPER HEADCANON)