Long before the days of paper, books in India were written on palm leaves. Palm leaves can be preserved only for a few years and people needed to rewrite the content of a set of palm leaves to another.
Some of such books that offer some light into the past of India and the knowledge that was available thousands of years ago are the Vedas, Upanishads and other literature.
Ashtanga Hridaya, Sushruta Samhita, Charaka Samhita and a host of ancient Ayurvedic texts were retrieved in the form of palm leaf scripts.
Practitioners of Ayurveda started the practice of jotting down their knowledge somewhere from 3,000 to 600 BC. Ashtanga Hridaya is younger than Sushruta Samhita and Charaka Samhita and is believed to be written in sixth or seventh century AD.
All the original texts were in Sanskrit language, which has translations to different Indian languages like Malayalam and Tamil.
The practitioners of different times have also published their observations and directions on the use of medicines and lifestyle directions. Thus Ayurveda literature today is a vast subject with books available in all specialties.