Books are allowed in Paper III, but searching through them takes a lot of time. If you start the paper by looking up answers, you may run out of time and leave easy questions unattempted. First attempt every question you can answer from your own preparation. Mark the doubtful ones, and open the books only in the last 30 minutes (after about 1 hour 30 minutes of the 2-hour paper) to check them.
Bare-act compilation of the allied laws tested in Paper III — Indian Contract Act, Transfer of Property Act, Hindu Succession Act, Companies Act, LLP Act and CPC.
General Financial Rules 2017 with GOI decisions, together with the compendium of rules on advances (house building, computer, festival and other advances) to Central Government servants.
Delegation of Financial Powers Rules with Government of India orders and decisions — financial powers of ministries, administrative departments and heads of department.
CCS (Conduct) Rules, 1964 with GOI decisions and clarifications — gifts, movable/immovable property returns, private trade, political activity and more.
The standard reference for FR/SR questions in Paper III. All four parts are needed for full coverage.
FRSRI
II
FRSRIII
IV