Not knowing your background, as HerrkaLeum pointed out, makes it more difficult to guage where you are. I'll assume you’re just getting into the field so bear with me. HVAC design is a learned process that takes a couple of years to get the hang of and many years to master. It certainly helps to have a background in thermodynamics, fluids, control theory, and heat & mass transfer. That said I haven't done a Laplace transform since college, and almost everything I've learned specific to HVAC was on the job training. If there is one thing, other than load calculations, an HVAC designer/engineer should understand it’s the psychrometric chart; I can't overstate how important this is.
This is not intended to be a plug but the ASHRAE handbooks comprise a great volume of information that is continuously updated based on the direction of the industry and new best practices. If you are just getting into the field, to save $$, you could probably pick up older versions as they are continuously updated every four years. The Fundamentals volume, as the name suggests, covers the fundamentals of the physical processes, including how to do a load calculation. Systems and Equipment and Applications are also very important as they capture how various systems interact with the building. There are other ASHRAE publications for fundamental HVAC topics (Load Calcs, Psychrometrics, etc...) and more specific subjects.
Another good source are PE exam study books. I refer to the Mechanical Engineering Reference Manual often. Somewhere in I here I should also mention your local mechanical, energy, and plumbing codes. There is more than one way to do things, your local code helps ensure its being done safely and efficiently.
There are many other good publication sources. My final suggestion is to find a Professional Engineer in the HVAC field to mentor you. You may have local ASHRAE, GBC, and ASPE chapters where you can meet people in the industry.
Good Luck