Short answer: Read NCERTs first in a fixed order, make short notes, then add standard reference books subject by subject. This gives a base before the heavier books.
Why NCERT first
NCERT textbooks explain basics in plain language and give you the vocabulary of each subject. Reading them first makes the later reference books much easier to follow.
A simple order
Many aspirants read history and geography first, then polity and economy, and keep science and environment for the end. The exact order matters less than finishing a subject before starting another.
Notes that help
Write brief notes in your own words, with headings that match the syllabus. Revisit them regularly instead of rereading whole books.
Which editions to buy
For NCERTs, use the latest editions. For reference books, check the edition year since policy and data change. Older copies are fine for static topics such as ancient history, but check before buying.
Related: affordable UPSC books.
Frequently asked questions
Do I need to read all NCERTs for UPSC?
Many aspirants read the NCERTs relevant to the syllabus subjects. Choose according to your optional and your current knowledge.
Are second hand UPSC books okay?
For static subjects usually yes. For current affairs and data-heavy subjects, prefer the latest edition.