Amit, if you look at CDS syllabus, it is the easiest and can be cracked with little but focussed preparation at home even without coaching.

Under English, topics like Antonyms, Synonmys, Spotting Errors, Sentence Improvement, Word Substitution, Selecting words, comprehension, ordering of sentences are covered. So you can practise these from books like:

Objective General English by RS Agarwal

Word Power Made Easy  by Normal Lewis

High School English Grammar and Composition

Under GK section subjects like Economics, Politics, History, Geography, BBiology, Physics, Chemistry, Defense related, Environment, Cultural, Sociology, Awards, Sports and Books are covered. SO you need to study Manoram year book and Lucent's General Knowledge.

Mathematics questions are elementary mathematics topics like Arithmatics, Algebra, Trigonometry, Geometry, Statistics. These can be learnt from 100th class books or a tution class just for 10th Mathematics. With this affordable coaching and preparetion, you can crack CDS exam.