Short answer: Many government exams share quantitative aptitude, reasoning and English, so buy a common core once and add only exam-specific material.
Find the overlap
Compare the syllabus of the exams you are targeting. Quantitative aptitude, reasoning and English are common, so one good book for each often serves several exams.
Add exam-specific books
Add general awareness and any subject-specific book only for the exams that need them. Use the official notification as your guide, not a long list from a forum.
Practice sets
Previous year papers and sectional tests build speed. Buy fresh copies of test books you will attempt, and use second hand copies for reference.
Frequently asked questions
Do I need separate books for each exam?
Not always. Common sections can be prepared from the same books, with extra material only for exam-specific topics.
Where can I find second hand government exam books?
You can browse second hand exam preparation books on Bookaao by category.