Start it with short books and short topics that are easy to absorb, memorize, and write. Generally, it’s a confounding idea to tackle large and thick books for clearing UPSC mains exams like essay paper. I came across UPSC Civil Services General Essay by Vidyasagar Gujje, it is a short book that has around 25 topics covered in short and concise manner. The book is so lucidly written that absorbing its content is very easy and even in two to three readings one can rewrite or better write the topics given in it. Always start with short topics so that you can get in the habit of reading and writing easily. And that is the main aspect of the exam preparation.