Salam,

The best introductory book will be" Dayal Sharma's Europe Since 1789".Take the mindful look at the previous papers and you would know the pattern.To me,the most handy book throughout the length and breadth of your journey through European History should be European History since 1789 by V.D Mahajan.
For an utterly exam-centric preparation,after you have had enough background of it all,you had best refer 'European history since 1789 by A.J Agarwal' in question-answer form.

People would certainly suggest you Europe since Napolean by David Thomson,a book worth a try.But it is upto you whether you can afford that much amount of time.It would be,however, rather unwise to just get engrossed into one subject at the altar of missing out completely on the other.

Having to pass CSS requires balance between all the subjects so do be mindful of the fact if you think you have little time at your disposal.
I sincerely hope you would come up with better strategy as you progress.
Wish you best luck..

AH,
Zohaib