For CDS, I would suggest you go through Arihant Path finder book. English & Maths is very simple in CDS. For General ability read maximum about Modern history & constitution of India.

One doesn't need to buy latest edition, because static topics (history, geography, science) remain one and same in all edition. So grab second hand/used copy for a cheaper price or borrow it from a friend. Any publication will work: Unique, TMH, Spectrum etc.

Practice previous sample papers, give mocks and tests.

Analyse your preparation and give a right path to your preparartion.

Good Luck.