What books for thermodynamics should I refer to for Gate preparation?

You should prefer books according to the following points given below.

• The books should be easy to understand for the candidates to read and perceive.
• The explanation should be straightforward, crisp, and easy to grasp. It should have correct data regarding the chapters and topics present within the curriculum of GATE.
• It will be good to go through the opening chapters of a few books from a library and pick the one that you can understand easily.
• The Internet is also a great place to find definitions and examples. If you are confused with a concept, google it, you will find easy explanations. 

Given below are a few most-recommended books for Thermodynamics. 

• Book for problem-solving PK Nag.
• Book for theory and concept A. Cengel and Michael A. Boles. It is a good book for the preparation of the interview too.

It is the same as teachers. Different books explain the same thing in different ways, you need to find which works best for you.