Frankly speaking the preparations for such exams are to be done right from when the student is in class IX or X. It has to be gradually done on a long time frame to absorb a lot of things which are considered useful for this exam. Anyway if that could not be done does not mean that now one can not achieve it. A person can achieve his or her targets in life at anytime by hard work and focussed approach.
Today there are many help books and reference books available in the market but they only act as a guidebook only and the real preparation is the reading and absorption of the read material by the aspirant. A good memory is a boon in such cases though a person having lesser memory can compensate it with repetitive studies. Making short and bulleted notes is one of the tricks people use for these exams. Reading newspaper and competition related periodicals is also a thing which many people recommend to the students.
Though there are many books recommended for the most coveted UPSC exam but I will recommend the following ones due to their quality and content -
1. Indian Polity for Civil Services
Examinations by M. Laxmikanth.
2. The Constitution of India by P. M. Bakshi.
3. Oxford School Atlas by Oxford Publishers.
4. Certificate Physical and Human Geography by Goh Cheng Leong.
5. Indian Economy by Ramesh Singh.
6. Economic Survey by Ministry of Finance.
7. India Year Book.
8. Ancient India by RS Sharma.
9. A Brief History of Modern India by Rajiv Ahir.
10. General Studies Paper 2 Manual by MHE (CSAT).
11. NCERT Books on various subjects.
12. NIOS Study Material for Environmental Studies.
13. Magazines like Yojana and Frontline.