In origin, an enchiridion is literally a small thing for holding in the hand, from Greek enkheiridion, which is made up of the parts en-, within, plus kheir, hand, plus the diminutive suffix -idion. In English, it has become a name for a handbook or concise treatise.
A famous example of a treatise with this name is the one St Augustine wrote around the year AD421, the Enchiridion de Fide Spe et Caritate (a treatise on faith, hope, and charity), in whose title appears the Latin form of the original Greek, the version that English has borrowed. Another example was penned by Erasmus in 1503, Enchiridion Militis Christiani, in English “Handbook of a Christian Knight”.
Outside references to such works, the word is extremely rare. It does appear in the SF novel Shadowfires by Dean R Koontz:
Sharp had remade his reputation by the manipulation of electrons, and Eric Leben had attempted to remake himself from a corpse into a living man by the manipulation of his own genes, and to Sharp it was all part of the same wondrous enchiridion to be found in the sorcerer’s bag of twentieth-century science.