In the old days, book distributors and sankirtan devotees usually carried some mantra cards like the one above to distribute. Hopefully this practice is still going on, because if there was only one piece of literature being distributed, it should be the maha- mantra, or the great mantra for deliverance.
Following in the footsteps of Lord Caitanya Mahāprabhu, the Kṛṣṇa consciousness movement is distributing the Hare Kṛṣṇa mahā-mantra and inducing people all over the world to chant. We are giving people an immense treasury of transcendental literature, translated into all the important languages of the world, and by the grace of Lord Śrī Caitanya Mahāprabhu this literature is selling profusely, and people are chanting the Hare Kṛṣṇa mahā-mantra with great delight. This is the preaching process of the Caitanya cult. Since the Lord wanted this cult preached all over the world, the International Society for Krishna Consciousness is acting in a humble way so that the vision of Śrī Caitanya Mahāprabhu may be fulfilled all over the world, especially in the Western countries. (from purport to Adi-lila, 6.19)