Which language was the religious book of the Jains written in ?

Jain religious texts were written in Ardhamagadhi Prakrit, not classical Sanskrit or Pali. Prakrit was the language of the common people in ancient India. While 'Prakrit' is the broader language family, the specific dialect used for Jain scriptures was Ardhamagadhi. Pali was used for Buddhist texts (Theravada canon). Sanskrit came later for Jain philosophical commentaries.