Hi,
Normally recommended book is that of Sadat Ali Shad that is called the Bible of English for CSS Candidates, else you can use Abdul Basit Jasra's book or R.H. Dhilon's book on precise writing. Jasra's book contain solved precises & R.H. Dhilon contain partially solved and rest unsolved precise.
You can also use the past year papers to practise and post your precise on this forum to get it check & corrected.
Visit this threads for precise writing:
http://www.cssforum.com.pk/css-compu...s-writing.html