dear aspirant,

NCERT books are really helpful and sufficient when it comes to NTSE preparation moreover the   other books that might be helpful are,

 R. S. Aggarwal Mental Ability books about 60%(or even more) questions come common from it, for MAT part.LCT part - NO NEED TO STUDY SEPARATELY.Still if you are very weak in English solve Arihant practice papers .That would be more than enough.

hope it helped.