The Syllabus for Zoology as an optional paper in the Mains examination is here https://www.civilserviceindia.com/subject/Zoology/main-syllabus.html Please go through the entire syllabus carefully and throughly to build better strategy for the preparation of the Main examination.
The syllabus includes various components and you need to do intensive reading for preparing this paper. As you know the marks of the optional paper is crucial to decide your ranking in final list, you to have to perform exceptionally well in the paper of your choice. The right strategy should be to go through each section of the syllabus and pick up the books given here and read them carefully. First try to pick up lighter books to clear your concepts and then go on to read the advance books. Dont try to do selective reading and gamble as there is only one optional paper now. Cover the entire syllabus with the list of books recommended and as you read the portions make notes simaltenaiosly after reading each segment.
Remember that your notes will be the final course material for reading and revising in future so take time to do a good job with it. So go ahead and start your preparation with books listed here. You can look for other refrences, if any section of the syllabus is not covered in these books.