A book carried in the hand for reference, especially one used for music or theology. Etymologically, it's a book that is meant to be able to be carried in one's hand, which today probably encompasses most books, but, ironically enough, not all 'handbooks.'
Greek, from en (in) and cheir (hand)
One of the most famous of all the enchiridions was the Enchiridion of Saint Augustine (November 13, 354 – August 28, 430), Bishop of Hippo Regius. His handbook was also titled 'Faith, Hope, and Love,' and was a written guide to the three graces of Christian worship, known as the Apostles' Creed.
Jason wished he had have read the cover a little more carefully - he knew it was too early for a third Assassin's Creed game.