1. Checkland, P. (1999). Systems Thinking, Systems Practice: Includes a 30-Year Retrospective. John Wiley & Sons. In Part 2, Checkland extensively discusses the distinction between 'hard' systems thinking, applicable to well-defined, technical problems (aligning with hard processes), and 'soft' systems methodology, designed for complex, human-centric problem situations (aligning with people processes).
2. Harmon, P. (2014). Business Process Change: A Business Process Management Guide for Managers and Process Professionals (3rd ed.). Morgan Kaufmann. In Chapter 2, Harmon discusses the nature of business processes, distinguishing between highly structured, repeatable work and unstructured, knowledge-based work, which corresponds to the hard process vs. people process classification.
3. Ould, M. A. (2005). Business Process Management: A Rigorous Approach. British Computer Society. Chapter 3, "What is a Process?", differentiates between processes that are highly deterministic and automatable ('hard') and those that are dependent on the expertise and discretion of people ('soft' or 'people' processes).