The Precious Key to the Secret Treasury

In the Early Heian Period (830) Emperor Junna ordered the representatives of each school of Buddhism to submit a treatise summarizing the doctrines of their respective schools. The six treatises presented on this occasion included the bulky Treatise on the Ten Stages of the Mind (Jū-jū-shin-ron), Taishō 2425, by the present author, which in ten fascicles details the doctrines of the Shingon School. However, this work proved to be too voluminous in comparison with the works presented by the other schools, and Kūkai was requested to submit a simplified version. The result was The Precious Key to the Secret Treasury

Kūkai’s method in clarifying the position of the Shingon School was to formulate a system of critical categorization made up of “ten stages of the mind,” embracing not only the various schools of Buddhism but also the religions of India and China; the Shingon School is ranked as the highest stage of all. It should be noted that The Treatise on the Ten Stages of the Mind is also known as the Expanded Treatise; in contrast, the present work is referred to as the Condensed Treatise.

Source
Jp. Hizōhōyaku (秘藏寶鑰), by by Kūkai. 3 fascicles.

On the Differences between the Exoteric and Esoteric Teachings

This treatise, On the Differences between the Exoteric and Esoteric Teachings, also known simply as Treatise on the Two Teachings.. In his comparative study of the qualitative differences between Exoteric Buddhism and Esoteric Buddhism, Kūkai discusses the Buddhas said to have revealed the two teachings, the contents of the teachings, the period of time necessary for attaining Buddhahood, and the benefits deriving from the two teachings. With the help of quotations from many scriptures, Kūkai demonstrates that Esoteric Buddhism is superior to Exoteric Buddhism in regard to all of the above points.

Source
Jp. Benkenmitsu nikyō ron (辨顯密二教論) by Kūkai. 2 fascicles.

The Meaning of Becoming a Buddha in This Very Body

In the Buddhism practiced in Japan prior to the time of Kūkai it had been maintained that it was possible to become a Buddha only after spending an enormously long period of time repeating the cycle of birth and death. This process known as “becoming a Buddha in three kalpas” (Jp. san-kō-jō-butsu) or “becoming a Buddha after passing through countless kalpas” (Jp. ryaku-kō-jō-butsu). In the present work Kūkai expounds the idea that it is possible to become a Buddha in this very body (Jp. soku-shin-jō-butsu). This work is thus an elucidation from the standpoint of the Shingon School of the theory and practice leading to such attainment. 

Source
Jp. Sokushin jōbutsu gi (即身成佛義) composed by Kūkai. 1 fascicle.

The Meanings of Sound, Sign, and Reality

Within the doctrine of the Shingon School there is the theory that the activities of body, speech and mind are essentially of the same nature as the corresponding activities of a Buddha. These three forms of activity are thus referred to as the “secret of body,” “secret of speech,” and “secret of mind,” and are known collectively as the “three secrets.” The present work, The Meanings of Sound, Sign, and Reality(Jpn. Shō-ji-jis-sō-gi) deals with the “secret of speech.” Quoting from the Mahāvairocana-sūtra (Taishō 848), Kūkai states that “sound” and “word” are in essence manifestations of the virtues of Mahāvairocana, the embodiment of truth itself, and that therefore mantras (Jpn. shingon) are actually manifestations of truth. The name Shingon School derives in fact from this view that the word is truth itself.

Source
Jp. Shōji jissōgi (聲字實相義), composed by Kūkai. 1 fascicle.

The Meanings of the Word Hūṃ

In Sanskrit, hūṃ (transliterated in Japanese as un) is the last letter of the alphabet in contrast to a, the first letter. The Meanings of the Word Hūṃ discusses the superficial and profound meanings of this letter, and is regarded as compulsory reading in the Shingon School. The superficial meanings of hūṃ are elucidated from the standpoint of Exoteric Buddhism, whereas the profound meanings are clarified from the standpoint of Esoteric Buddhism.

Source
Jp. Unjigi (吽字義), composed by Kūkai. 1 fascicle.

The Illuminating Secret Commentary on the Five Cakras and the Nine Syllables

Towards the end of the Heian Period, when the author of The Illuminating Secret Commentary on the Five Cakras and the Nine Syllables lived, the belief in rebirth in the western paradise of Sukhāvatī was gaining in popularity. In this work Kakuban sets out to establish, from the standpoint of Esoteric Buddhism, that Mahāvairocana and Amitāyus are in fact one, that the Ghanavyūha paradise of Mahāvairocana and the Sukhāvatī of Amitāyus are the same place, and that rebirth in Sukhāvatī is equivalent to the attainment of Buddhahood. The “Five Wheels” mentioned in the title refer to the five constituent elements of all terrestrial phenomena (earth, water, fire, wind and air), and the “Nine-Letter Mantra” is Amitāyus’ mantra which consists (in Sanskrit) of nine syllables. Kakuban demonstrates that since these five wheels and nine letters are identical, Mahāvairocana and Amitāyus are also in fact identical. This work is thus regarded with importance in that it is an exposition of the esoteric view of Amitāyus and Sukhāvatī.

Source
Jp. Gorin kuji myōhimitsu shaku (五輪九字明秘密釋), composed by Kakuban. 1 fascicle.

The Mitsugonin Confession

At the age of 41 Kakuban, the author of The Mitsugonin Confession (Jpn. Mitsu-gon-in-hotsu-ro-san-ge-mon) and the founder of the New Doctrine branch of the Shingon School, resigned all his official posts and, shutting himself up in a room of the Mitsugon-in Temple, spent 1,500 days in silent meditation. The present work was written at this time and is in verse form, consisting of 44 lines of 7 characters a line.

It is an expression of what might be called “absolute penitence,” in which the author repents of not only his own sins but also those committed by others. Deploring the moral corruption of the monks at the time, Kakuban probably adopted this style in order to sound a warning to the Buddhist community. Even today this work is recited at least once a day by monks belonging to sects descending from Kakuban’s school.

Source
Jp. Mitsugon inhotsuro sange mon (密嚴院發露懺悔文), composed by Kakuban. 1 fascicle.