In her acknowledgments to The Nun, Simonetta Agnello Hornby thanks Enrichetta Caracciolo (1821-1901), whose I misteri del chiostro napoletano (Mysteries of the Neapolitan Cloister) was useful for, among other things, "descriptions of ceremonials" (327). That's rather understating the case. In fact, The Nun relies very heavily on Caracciolo's account for its plot and several incidents, but yokes it to a romance narrative that echoes eighteenth- and nineteenth-century British fiction, especially Pride and Prejudice (which the protagonist reads) and Jane Eyre (which the protagonist doesn't). One way of thinking about The Nun is that it upends nineteenth-century critiques of novel reading: Agata, our heroine, has little in the way of agency, thanks to the structure of mid-19th c. upper-class Sicilian society, but she is both liberated and seduced by novel-reading--something the novel celebrates as freedom from stultifying constraints, rather than moral corruption.
Like Mysteries, The Nun is set against the backdrop of the Risorgimento, although it ends earlier, in 1848. Agata's life plot adheres roughly to Enrichetta's: both women are daughters of military fathers who married women in their early teens; both develop a passion for a young man whom they glimpse primarily from a balcony, and who is destined for another, wealthier woman; both discover that their own mothers (as well as the youth's parents) object strongly to the potential match; and both find themselves unceremoniously deposited in a Benedictine convent after their father's death. In both cases, the mother leaves Enrichetta/Agata in the convent with a promise that the ordeal will last for only "two months" (25/100) if Enrichetta/Agata dislikes the experience; in both cases, the mother returns, supposedly to remove her daughter, only to abandon her instead, which leads the daughter to collapse. Both women go on to become Infirmarians. Both have an aunt, also professed, who suffers increasingly from dementia and/or insanity, and is treated horribly. Besides this basic plot outline, The Nun also borrows a number of incidents and set pieces from Mysteries, including the poor babies in the procession car; the protagonist's unexpected invitation to the convent; the quarrel over wearing curls vs. straight hair before being admitted to the convent; the tour of the convent itself; being fondled by the priests; the anonymous Protestant who objects when the protagonist's hair is about to be cut; singing with her insane aunt; the concealed tumor; and the jewel theft. James Garson, Agata's English beloved, is inspired by an English captain whom Enrichetta meets early on in her memoir. And so on.
Now, that being said, The Nun diverges sharply from its source on the count of politics. Enrichetta's personal development maps onto her growing nationalist awareness, what she describes as a "different and clearer light of salvation" (Mysteries 114); she translates her rebellion against her incarceration in the convent into allegiance to the new political cause, linking the monastic system to "despotism" and praying instead for "the downfall of tyranny, and the triumph of the nation to which it was my boast to belong" (Mysteries 114). Instead of focusing on her putative destiny as wife and mother, Enrichetta imagines herself part of a larger women's movement devoted to the Italian cause, possessed as she is with a "sacred love of country" (Mysteries 146). For Enrichetta, nationalism is civic religion, a more-than-worthy substitute for the stultifying and infantilizing qualities of monastic life enjoined by Catholicism. The convent represses authentic femininity; nationalism, by contrast, liberates it into a new fullness. The narrative climaxes, in fact, with Enrichetta associating the end of her time as a nun with Garibaldi's entry into Naples, so that personal and political freedom are ecstatically united. And, as an afterthought, she gets married--a comic ending to her plot, but one that is very much not represented as runaway grand romance. By contrast, The Nun appropriates most of Enrichetta's life for Agata, but assigns the high political aspirations to Agata's sister Sandra, who embraces her husband's argument that "a patriot's woman must remember that certain sacrifices are necessary in order to attain a given higher end: the unity of the nation and the good of the Italian people" (The Nun 139). But the reader cannot help noting that despite the apparent egalitarianism of Sandra's marriage and her radical political affiliations, she winds up deeply unhappy, thanks to her husband's eventual philandering; nationalist sentiment, far from going hand-in-hand with women's liberation, turns out to enable yet another mode of useless female self-sacrifice. Instead, the novel endorses the free pursuit of romantic desire as woman's only route to entire self-fulfillment. Although we do encounter a couple of entirely content nuns, especially Donna Maria Giovanna della Croce, Agata never shares their vocation and finds their counsels inadequate to her need "to fall in love, be fecund, bear children" (The Nun 181). Enrichetta's quest for political and personal liberty thus transforms into a quest for sexual self-realization, one free from the constraints of financial shenanigans (the politics of dowries and arranged marriages) and inequality (the nun who dies bearing a priest's child). In the world of the novel, such relationships appear to happen almost by accident, as in the case of Agata's mother's second marriage (originally contracted for pecuniary reasons) or on the margins (the lesbian lay sisters). The personal overtakes the political. Unlike Sandra, Agata welcomes the possible coming of a "better, free world" (223) primarily for her own emancipatory options. It is here that novel-reading comes into play.
Early on in Mysteries, Enrichetta is horrified to find that a priest has given a young nun a copy of Denis Diderot's La Religieuse (another influence on Hornby's novel, starting with the title), which she describes as "full of the most revolting improprieties" (54). The Nun takes the obviously erotic connotations of this gift and turns it into a form of re-emplotment: Agata's relationship with James Garson is textual long before it is ever sexual, and turns on the long-running exchange of novels and poetry (from him) and commentary (from her). Even when James isn't sending her novels, she is reading them on her own, with a noticeable preference for romances like Corinne (The Nun 188) and "tragic, heart-breaking love stories" (The Nun 182). In a Victorian novel, her reading preferences would mark her out as doomed to an early death at the hands of an evil seducer; here, the wildness of the passionate plots she so adores proves personally liberating by celebrating the power of grand passion (even when it ends unhappily) instead of erotic and personal self-repression. James' first gift to her, Pride and Prejudice, implicitly offers her a new way of understanding her own life, especially in the wake of her revulsion from her first beloved, Giacomo; rereading the novel and falling "head over heels in love with Darcy" (The Nun 178), which leads her to restart her exchanges with James, is the sign of her emotional maturity. After her profession, she conceals her comments on James' gifts in the paperoles--miniature altars--that she makes and sends to him, making self-expression substitute for an image of the Eucharist at the heart of her miniature. Her literary criticism remains secret even to the reader, an alternate form of sacred communion that derives from the shared world of books and excludes everyone but James. Later, James invites her to read along with him by marking lines with his initials, using literature to ventriloquize his passion (The Nun 276-77). Imaginative reading-with-another offers a double means for Agata to escape her mental and physical imprisonment, both by identifying with creative literature and its characters and passions, and by sympathetically engaging with James' own interpretive processes. Notably, their exchanges are a form of secret writing, which, unlike Enrichetta's texts, are difficult to decode and often hard to trace--indeed, cannot always be understood as "communicating" at all. This private language, stitched together from novels and hidden letters, stands apart from the political discourses at play in the worlds of Sandra's husband Tommaso and Enrichetta herself, which are openly subversive and dangerously readable by the authorities. Literature provides the figurative space in which James and Agata can meet, but it also suggests ways in which Agata can, as it were, escape the confines of Enrichetta's memoir.
This strategy climaxes (in more ways than one...) in James' rather astonishing present to Agata of M. G. Lewis' The Monk, which clearly stands in for La Religieuse. The novel, James explains in his accompanying letter, is "full-blooded and carnal. Like the relationship that I desire with you" (The Nun 264). Although we never find out what, exactly, Agata makes of The Monk, Hornby does borrow tropes from anti-Catholic Gothic (the suicidal nun, the insane nun, solitary confinement, sexual abuse at the hands of priests) that are more subdued in her source material, and the outcome of The Monk's secondary romance plot prefigures Agata's eventual reunion with James. More to the point, James' forthright declaration of the novel's sexual intent precipitates Agata out of Pride and Prejudice and into Jane Eyre, as James is already married and, like Rochester, proposes that they live together without officially cementing the relationship. James' proposal, however, is an egalitarian one: "you will always have my love, ample economic independence, and the life that you want to live, where and as you wish" (The Nun 263). This is not a Rochester who seeks to absorb the life of his mock bride, but a proto-feminist offering up an alternative family form; unlike Sandra's husband, he celebrates mutual liberty and freely-chosen obligations instead of absolute self-subordination to a political ideal. Whereas Enrichetta yokes femininity to nationalism, Agata finds it instead in the union of Godly and erotic love, first in her sexual encounter with James and then later, at another convent, when she concludes that "there was really no difference between sacred love and profane love" (The Nun 296). Divinized eroticism (despite pesky issues like marriage, adultery, etc.) supplants political action--or, to put it differently, when faced with a choice between history and romance, Agata comes down firmly on the side of the romance.