Literacy is not absolutely essential to attain the end of lectio as listening to readings and homilies, lectures and conferences and reciting psalms and other texts by heart can make up for actual reading. Periods of Community reading as at Mass, Divine Office, meals and before Compline (the final Hour of the daily Divine Office) have always been practiced in religious communities.
Over the years the amount of such reading adds up to a surprising amount of time. Community reading also ensures that the more desirable books are available to a greater number of people. In recent centuries in the Cistercian tradition, in some monasteries, reading was done in the Scriptorium (writing room). Reading in common, even though each one may read their own book in private, does add something to the exercise.
It is interesting that neither scriptorium nor library were budgeted for in the traditional Cistercian monastic plans. So to gain the benefits of lectio divina it is not absolutely necessary to be able to read. Every Mass, every sacrament, is accompanied by Scripture readings so by simply listening attentively to them we are doing lectio. Some of the medieval scribes were illiterate and for that reason were hired to copy confidential documents.