There is no any specific books for JNU Entrance Exam, You can refer the previous year papers to know the level of the questions. You should have the basic knowledge of the subjects like History, Economics, International Relations, Philosophy, Indian Politics. You can also refer the books for the subject:

The Globalization of World Politics - Baylis & Smith

India after Gandhi - Ramachandra Guha

Indian Government and politics - A S Narang

International Relations in the 21st Century - Pushpesh Pant

Global Politics - Andrew Heywood

Good Luck!