If you are looking for specific "hot" current trends in HVAC—such as smart thermostats variable refrigerant flow (VRF) systems, or geothermal heat pumps

The primary resource for HVAC studies by Anant Narayan (P.N. Ananthanarayanan) is Basic Refrigeration and Air Conditioning

: It is designed not just for exams but for "service men" and practicing engineers who need to solve real-world problems, such as leak tests, vacuuming, and gas charging. System Components