ShallowHierarchy

Our experience is that systems we produce this way tend towards very flat class hierarchies. This avoids well-known problems, such as the Fragile Base Class [12], which make systems harder to understand and modify.

—"Mock Roles, Not Objects", Freeman, Price, et al.

It is also notable that the Feynman lectures (3 volumes) write about all of physics in 1800 pages, using only 2 levels of hierarchical headings: chapters and A-level heads in the text. It also uses the methodology of sentences which then cumulate sequentially into paragraphs, rather than the grunts of bullet points. Undergraduate Caltech physics is very complicated material, but it didn't require an elaborate hierarchy to organize.

—Edward Tufte