To prepare for Computer Science Optional Paper in West Bengal Civil Services examination, first you should make a proper routine that you'll follow throughout your preparation. Then go through the entire syllabus.

• Computer Science Paper 1, syllabus-

Algorithms, Number systems and Arithmetic, Theory of Counting, Graphs and Algorithms, Boolean Algebra, Models of Computer Machines, Numerical Algorithms, Operations Research, Circuit and Network Theory, Basic Electronics, Instruments, Digital Logic and Systems, Data Communication, Data Structure.

• Computer Science Paper 2, syllabus-

Operating System, System Analysis and Design, OOPs, Computer Architecture and Organization, Micro Processor, Computer Networks, DBMS, Assembler, Loader and Linker, Compiler, Graphics and Multimedia.

For preparation, Arihant's Computer Awareness is one of the best books recommended by top educators, other books like Computer Science and ApplicationsbyChandresh Shah. These are easily available on e-commerce sites.