I've finished my Bsc in Politics and International Relations from University of London (international programmes) and I'm hoping to appear in the CSS exams in 2020

The optional subjects I've chosen are:
1)International Relations
2)Statistics & Geology
3)Business Administration
4)European History
5)Environmental Sciences
6)International Law
7)Geography

Has anyone attempted any of these subjects? I would love to hear any tips that you could share!
I've been going through the past papers and all but I'd like to know if there are any specific books you'd recommend I go through, or should I be reading all the recommended readings mentioned in the syllabus?

Any help would be so appreciated!
Thank you in advance