For working candidates, it is little tricky to get time for preparation, hence you need to plan your engagements effectively.
The Exam-Plan is such that you can’t afford to look at this examination stage-wise. So, you need to adopt an integrated approach for preparation, particularly for those constituents that are common in both Preliminary as well as Main Examination.
Besides, no single book can cover the vast syllabus that encompasses the constituents from diverse disciplines. Besides, the dynamic approach UPSC is adopting makes your task more challenging.
So, commence preparation with some basic books and NCERT books are the best resource. The conceptual clarity you will gain with these books will help you as you advance with reading standard text-books.
Above all, daily newspaper reading an effective use of internet would help you in keeping abreast with the latest.
Suggested Book List: Sociology