What are the study material should prefer for SO-HR?

The best way to prepare for the HR Specialist Officer is to check the syllabus by IBPS and find those topics in your HR books. That's a sure shot but the long way.

The other way is to get one of the books specially published for Specialist Examinations:

1) Buy IBPS Specialist Officer SO Spl-5 Human Resource H.R./Personnel Exam Self Study Guide Cum Practice Work Book Book Online at Low Prices in India  by Pratiyogita Kiran
2) Buy IBPS (CRP SPL-V) Specialist Officer HR Officer 2016 Study Guide Book Online at Low Prices in India by Arihant