Today, there are very few books on Shingon in the West and until the 1940s, not a single book on Shingon had ever been published anywhere in the world, not even in Japan. Since this lineage was brought over to Japan from Tang China over 1100 years ago, its doctrines have always been closely guarded secrets, passed down orally through an initiatic chain and never written down. Throughout the centuries, except for the initiated, most of the Japanese common folk knew little of its secretive doctrines and of the monks of this "Mantra School" except that besides performing the usual priestly duties of prayers, blessings and funeral rites for the public, they practiced only Mikkyō "secret teachings", in stark contrast to all other Buddhist schools, and were called upon to perform mystical rituals that were supposedly able to summon rain, improve harvests, exorcise demons, avert natural disasters, heal the sick and protect the state. The most powerful ones were thought to be able to render entire armies useless.
Even though Tendai also incorporates esoteric teachings in its doctrines, it is still essentially an exoteric Mahayana school. Some exoteric texts are venerated and studied in Shingon as they are the foundation of Mahayana philosophy but the core teachings and texts of Shingon are purely esoteric. From the lack of written material, inaccessibility of its teachings to non-initiates, language barriers and the difficulty of finding qualified teachers outside Japan, Shingon is in all likelihood the most secretive and least understood school of Buddhism in the world.