With great sociological and psychological insight Hegel says that "stoicism, the freedom which goes back into the pure universality of thought, could appear as a general form of the world spirit only in a time of general fear and servitude but also of general education, which had taught men to think" (Phenomenology of Spirit, 153). The point seems to be that the frustration of the freedom of act results in the search of a type of freedom immune to such frustration. Where the capacity for abstract thoughts exists, freedom, outwardly thwarted, is sought in thought. Hegel seems to suggest that the stoic's freedom is what Freud might call a substitute gratification.
However, this suggestion, interesting and true as it may be, is not completely consonant with the genesis of stoicism in the Phenomenology. Although stoicism arises from servitude rooted in fear, it does not arise because the servant is not allowed to act freely, but because all action proves ultimately futile.
it is still worthwhile to think about why this "insight" of Hegel might be thought perspicuous independent of the particular theoretical standing of "stoicism" in Hegel's system.
The prison writer is, like the Hegelian stoic, still a seeker of freedom but, unlike the Hegelian stoic, not one that considers all action futile. Rather he has come to see that actions are still available to him, even if not those that had previously been available to him as a fuller mode of physical expression. So, like the Hegelian stoic, he has moved from considering freedom to being a purely practical affair to being a "peculiarly theoretical and epistemological one" (Soll, 30) but one still grounded in the activity of writing.
Those that place prisoners in solitary confinement are onto a vitally necessary piece of knowledge for the oppressor: if confinement is to work as a mode of repression, it must aspire to as much totality as possible.
Update: Just chatting with Corey Robin over on Twitter, who suggested adding Gramsci, Solzhenitsyn and Bukharin to the genre of prison literature, and also noted the relevance of Hannah Arendt's remarks about totalitarianism to my last sentence. Good points of course; my small list above merely scratches the surface, and I would even supplement Arendt with the Orwell of 1984.