4 ideas
Every letter was composed knowing a prison official would read it first, so each one addresses two readers at once: the family member named and the authority deciding whether it passes. Reading the letters this way shows restraint as a deliberate strategy. Measured phrasing kept communication channels open that anger would have closed through withheld or destroyed mail.
Composure is treated as territory the prisoner can still defend when almost everything else is controlled. A guard who provokes an outburst has extended his power over the prisoner's inner state. Declining to react keeps that last area self-governed and denies the provocation its purpose.
Recording small daily disciplines turns a long, open-ended sentence into manageable units. That makes endurance a habit that is kept up, not a feeling waited for.
A polished autobiography presents composure as a settled result, while the raw letters and notebooks show the repeated effort, lapses and self-correction that produced it. To learn how a disposition was built, study the working documents rather than the final narrative. The final version smooths away the struggle that holds the transferable technique.