I am rereading Edgar Schein's Process Consulting vol. 1 from 1988 found here on Amazon. Unfortunately, this great text is $45 for a new copy. Despite its high cost, it's easily one of the best guides to leadership and consulting I've found, and I use his methods in Even Motionamp. Expansions on his core text are his vol. 2, and the more recent Humble Inquiry and Consulting guides.
Schein is very much a mainstream business guru. Known for his Iceberg model and work in organizational culture, this long tenured MIT professor focuses on "helping" in his consulting approach. "Helping" has evolved as a more precise term for Schein's techniques; as 1990s era Lean and Six Sigma reoriented "process" to mean more (at least notionally) mathematical approaches to consulting, Schein has worked to keep consulting as a broader and more collaborative concept. Schein's main focus is in joint work of all kinds with his clients. Much of this is interpersonal and psychological in nature. In future work, I aim to reconcile Schein's helping approach with Critical Performativity tensions discussed in Critical Management Studies.
In Schein's helping approach, the client and consultant work together to conduct joint diagnoses of organizational challenges, and construct joint solutions. The consultant's job is not necessarily to provide expert advice the organization lacks, but rather facilitate the discovery of problems and solutions that, for a variety of reasons ranging from leadership to cultural challenges, the organization struggles to solve on its own.
In my consulting, I avoid doctor/patient and expertise models of consulting that are more prevalent. These methods usually result in defensive clients who ultimately reject proposed solutions -- they don't own the ideas. Joint work however, using Schein's model, promotes a healthy client/consultant relationship which can endure through a variety of improvement and growth projects.