First Indian book a historical perspective:
Rigveda: Dating back to 1700-1100 BCE, the Rigveda is a collection of Vedic hymns in Sanskrit, considered one of the oldest extant Indo-Aryan texts and a cornerstone of Hinduism. This could be considered the "first book" in terms of historical significance.
Ramayana and Mahabharata: These epic poems composed sometime between 3rd century BCE and 3rd century CE hold immense cultural importance in India. While technically oral compositions passed down for generations before being transcribed, their influence on Indian literature makes them strong contenders for an early "first book."

From a specific language perspective:

    Tamil Sangam literature: Sangam poetry, encompassing works from the 3rd century BCE to the 3rd century CE, represents some of the earliest surviving Tamil literature. It showcases a rich poetic tradition within a specific language.
    Tirukkural: This ancient Tamil ethical treatise, composed around 3rd century BCE, holds significant philosophical and literary value. It could be considered the "first book" within the Tamil language context.