You can opt philosophy because it's not only high scoring subject but also valuable in providing sufficient data for English Essay Paper in the form of different intellectual and philosophical approaches (e.g Logical positivism, & Analytic Philosophy etc.) theories (e.g. Idealism & Materialism etc.) and methods (e.g. Pragmatism & Existentialism etc.) in relation to issues and problems in modern era.
Following books covers the complete syllabus for CSS Philosophy.
For Paper I:
A Concise Introduction to Logic by Patrick J. Hurley
For Paper II:
History of Philosophy Eastern & Western by Radhakrishnan
Studies in Muslim Philosophy by M. Saeed Sheikh

You can collect study material about rSub-continental Muslim Philosophers (Shah WaliUllah, Sir syed Ahmad Khan, Iqbal) from M.M Sharif's History of Muslim Philosophy Volume II.