For building the base in different subjects, NCERT and NIOS are the foundation sources. These books are meant to be read at a pace that allows you to understand the concepts and terms, both for UPSC Prelims and Mains. It is not required to read from both sources or get into Old NCERT vs New NCERT. Choose any of these and it will suffice. You just have to pick one that you find easy to decipher. After laying off a solid foundation in all the prescribed disciplines with these textbooks, revision tests must be attempted and then move ahead to other prominent sources for the exam.
We have compiled all the relevant UPSC books for the subjects per the Syllabus from classes 6th to 12th. You can easily download the NCERT books PDF by clicking on the download option given beside each one of those.
Verifying, please be patient.