Jexpo is a relatively easier examination & it don't require such early starting. So if you are willing to go for the JEXPO, the idea time for preparation will be from class 9th or 10th. This preparation will be enough. Now if you wish to strengthen your basis then you can study different foundation books but for jexpo those aren't required.

Some of the best books for JEXPO preparations is provided below:

1. Master Guide JEXPO West Bengal Polytechnic In Bengali

Author: Biplab Haldar, Pragati Dalal, Dr. Santosh Kumar Ghorai

2.Mathematics for Class 10

Author: R D Sharma

3.Polytechnic Jexpo (Bengali)

Author: Jayanta Gupta

4. NCERT 10 th Class Text Books

5. MCQ For Polytechnic Joint Entrance Examination (JEXPO) (Bengali)

Publication: Oriental Book Company Pvt. Ltd.

I hope this helps. Wish you a great eve!