If you want to get good marks in your UPSC and NDA exams it is very important for you to work on synonyms as well as antonyms. Many candidates prefer to look for synonyms and antonyms questions from previous year papers but it is not mandatory that those questions will land in your question paper. I am mentioning some good books that you can prefer for studying the questions of antonyms.
1. Antonyms, Synonyms and One Word Substitution by RAM SINGH YADAV
2. Synonyms and Antonyms Anglo by ROSHAN TOLANI
These are good books and have quality content for the preparation of competitive exams like UPSC, NDA, SSC CGL, etc.
Some of the books which you can prefer to study synonyms and antonyms from are -
1. Scholastic dictionary of synonyms, antonyms and homonyms
2. Oxford dictionary of synonyms and antonyms
3. Arihant publications - For all competitive exams Synonyms and Antonyms
4. BMW - brilliant master of words
Hope this helps.
All the best!