ISA is the home of control systems for process manufacturing and a wellspring of information on every kind of control technology and control system for discrete manufacturing and for infrastructure and utility and building control.
ISA is the best place to find information on control theory, from basic continuous control to mechatronics and discrete control theory. ISA has the most information for you on process control, batch control (ISA wrote the batch standard, ISA88), advanced control-- both model-based and non-model-based, simulation, and control systems and techniques.
ISA has textbooks, training, tutorials and experts available on any control system technology, from programmable controllers, motion control systems, to DCS (distributed control systems) through SCADA (supervisory control and data acquisition) systems, and all the hybrids that are used in manufacturing and process plants of all types, like boiler control systems, safety systems and burner management systems. If you need information on control systems, you can find it here.
ISA is the place for information about human factors in automation, from HMI design and operation and software standards, to alarm management systems. ISA wrote the standards for those, and for cyber security as well.
In addition, ISA has recognized that few places exist where you can get organized training and be certified so you can actually show what you know. ISA has developed certification for control system technicians (CCST), and produces the examination for Control Systems Engineering licensing. Recognizing that automation is as much a multidisciplinary art as anything\, ISA developed the unique Certified Automation Professional, or CAP training, examination and certification.