dear krishna ,

yes, b.com students can apply for combined defence service. the eligibility for cds ota-Degree of a recognised university or equivalent.In market you found many books of cds preparation but in my opening you can prefer cds pathfinder of arihant .but before going to book you must know about the pattern of the exam by seeing the previous year papers.

if you want to more information then you can go through the below link

https://competition.careers360.com/exams/cds

but remember after qualifying exam you have to face the ssb interview which includes physical fitness test as well as Personality cum Psychometric tests and Intelligence interview.