Hi Nishu,

For NRTI you can refer following books:

1.Aptitude: R S Agrawal. You can also go through sites like indiabix.

2.English: Wren and Martin, I think will be the best book for English. If you have ample time, please go through it once.

3.General Knowledge(Current Affairs): Most unpredictable thing I found in this exam. Depends on your prior knowledge. I found it mostly static GK from Geography. I But if you want to prepare it too, study Indian Express and some updated Yooutube videos will be helpful.

4.Mathematics: Nothing too hard in Mathematics, just the basic knowledge upto 10 class will be enough to crack this exam.

Hope it helps,

All the best!