Hello!
You can try the following books:
1. NATA & B.Arch Question Bank  by Ar. Shadan Usmani
2. NATA & B.Arch complete self study material: entrance examination by  Ar. Shadan Usmani
3. A Complete Self Study Guide for B.Arch by P. K. Mishra
4. B.Arch Entrance Examination Guide by Surendra Kavimandan

Hi Harsh,

Although not sure about purchasable books for JEE Mains paper 2, but I can include the following useful blog links that could help you get downloadable materials as well as for online study

Also You can get the useful materials and study topics here in this page - Resources for JEE Mains paper-2 (B.Arch/B.Planning)

You can get the complete list of syllabus here - JEE Mains Paper-2 (B.Arch/B.Planning) Syllabus

Also, the same blog can be helpful for NATA too except that Math and sketch part is not applicable! check the basic differences in syllabus