Given the importance of motherhood in the narrative arc of Toni Morrison's Beloved, a good choice of theorist would be Julia Kristeva. Her work balances deconstruction and psychoanalytic theory with postmodernist feminism. Several of her most important theoretical works focus on motherhood, which will be important for analyzing the central releationship of the novel between Sethe and Beloved.
One particularly important essay by Kristeva for your project will be "Stabat Mater," one in which she attempts to revision the concept of the maternal, deconstructing both the traditional Christian notion of the maternal as the self-sacrificing image of the Virgin Mary, the Pietà who remains mourning the death of Christ, and the Freudian construct of the mother as an embodiment of the desires of primary narcissism.
The relationship between Sethe and Beloved, in which the child in a sense consumes the mother's identity, could be analyzed in terms of Kristeva's theory of the abject, a combination of desire and horror at the dissolving of self into object, and an eruption into the present of the pre-objectal relationship, before mother became separated from daughter.