Lurking this thread and taking notes. Some of these interpretations are blowing my mind. I never caught half of that subtext. Time for a re-read, I guess.
The letters are woven throughout 'The Queen of Nothing' and 'How the King of Elfhame Learned to Hate Stories.' Honestly, reading them in isolation loses so much impact.
Each one arrives at a crucial emotional beat in Jude's journey. Getting them via a list strips away the context of what she's facing when she receives them.
I'd strongly recommend just rereading the final book and the companion novella. The pacing and reveals are part of the experience. The anticipation between chapters is part of the fun.
You miss the gut-punch of finding one when Jude is at her lowest, or the hope it sparks. They're narrative devices, not just collectibles.
The concept of 'deserve.' Jude believes she must earn everything, that she deserves nothing by birthright. Cardan, born with everything, feels he deserves nothing because he never earned it. His letters reveal a profound sense of illegitimacy. His attraction to her is partly because she embodies the 'earning' he feels he lacks. Her struggle justifies her existence in a way his privileged birth never could for him.
They keep the conflict alive. Without the letters, Jude's exile could have become a clean break. The letters are the pinpricks that prevent the wound from healing. They're a constant, low-grade irritation and fascination. Their entire relationship is built on a foundation of conflict, so the romance, when it comes, has to grow from that same soil. The letters ensure the soil stays fertile by regularly turning it over.