I am a beginner in Microsoft D365, I want to map my price field from "book entity" to price field from "reader entity" as soon as book is selected from lookup on reader form, how can i achieve this using JS?
You don't have to create any JS for this. You can use OOTB functionality to do this. First you need to create a relationship between this two entity then you can easily create a mapping between this two field inside the relationship.