Hannyashin-gyo Hiken (般若心経秘鍵)
"Hannyashin-gyo Hiken" was a commentary on "Hannya Shingyo" (the Wisdom Sutras) and was written by Kukai around 818.
Some schools of the Shingon sect sometimes read "Hannyashin-gyo Hiken" as makura-gyo Death Guidance (pillow sutra).
Content
It points out the similarities and differences between Kumaraju's translation of "Hannya Shingyo" and Daihenkaku Sanjo's (Xuan Zang) translation and it also defines "Hannya Shingyo" as a teaching to dive into deep meditation with the help of Daishin Shingon mantra of Hannya-bosatsu.
It preaches that "Hannya Shingyo" should not be mixed with "Daihannyaharamitta-kyo Sutra," because they contain the same phrases, just as dragons shouldn't be considered the same as snakes because both have the same scales. It criticizes the perspective of considering "Hannya Shingyo" to be the summary of 600 volumes of "Dai Hannya Shingyo."
It also pays attention to Shingon granted to the last part and recommends that you should not read "Hannya Shingyo" but make it the object for kansho (meditation based on Shingon nenju) and practice it.