All I came to know about Āmnāya is that it refers to the 5heads of Shiva facing 5 direction and one being a secret (kālāgni Rudra) where each head revealed Āgama to Devi. So basically it means a sacred and secret tradition. Also I heard that abhiseka, āmnāya-krama and krama-deekshya plays a prominent role in a Tantrik practises for Self-Realisation. So where can I learn the concepts about them? Suggestion of good book would be a much help.