Meister Eckhart and the Beguine Mystics: Hadewijch of Brabant, Mechthild of Magdeburg, and Marguerite Porete
by Bernard McGinn
An insightful look into both Beguine mysticism and the history behind vernacular theology. This helped me better understand a lot of the concepts present in The Mirror. Will probs re-read The Mirror with a companion book, though.