this is a broad subject and it would be best to understand what kind of controls you want to learn about. If your looking at how DDC controls works with fire protection, like smoke detectors that is easy. Most of the BAS systems of today like, Honeywell, Carrier or Trane are plug and play. Ask for some owner manuals and Installation manuals and you can read through how they operate and what they do. Also, get your hands on some control asbilts and the sequence of operations from office buildings or plants.. That will go along way understanding functionality. Because if you jump into logic and all that at first, you will fry your brain. Once you understand how things work, how it is used and where etc.. you then can dive into logic and design or make changes for a particular application. One last thing, if you don't use it, you loose it.. I have an old Carrier parker system in a building.. when I don't keep up with it, it takes me a bit to remember how things work when I have a problem.