It is a section (Parva) in the Mahabharata Epic (Ithihaasa). Each section in the Mahabharata would also be considered as separate books in the whole volume.
According to the 18 sections classification, it comes under the Bheeshma Parva (Parva 6). According to the 100 sections classification, it is the Bhagavad Geetha Parva under the Bheeshma Parva (Parva 67).