Is there a (text)book on how academia works?

The study of the higher education and academia is a huge area of research, and I must admit, I am no expert in it. While some publications are opinion-based, as Massimo Ortolano notes in his comment, there is a growing number of research-based works on academia developed within different disciplinary frameworks, which includes those developed within the framework of academic sociology, as you mention in your question.

Among the trailblazing works in the sociology of academia were the publications by R. K. Merton, and also R. Whitley, The Intellectual and Social Organization of the Sciences (1984), and P. Bourdieu, Homo Academicus (1984), focusing on French academia.

Among the recent notable books in this field, I can cite M. Lamont, How Professors Think: Inside the Curious World of Academic Judgment (2010), based on a study of US grant peer review panels, and J. R. Posselt, Inside Graduate Admissions: Merit, Diversity, and Faculty Gatekeeping (2016), based on participant observations about a number of graduate admission committees at US universities. Tracing the differences between the branches (varieties) of academia and explaining them is a major theme in both these books. But there are many more.

Thanks @greenb for your link to Merton, I found a good pointer to everything about science itself: Science studies. One of the introductory books I found is David J. Hess, Science Studies: An Advanced Introduction. Here is its introduction:

Science Studies is the first comprehensive survey of the field, combining a concise overview of key concepts with an original and integrated framework. In the process of bringing disparate fields together under one tent, David J. Hess realizes the full promise of science studies, long uncomfortably squeezed into traditional disciplines. He provides a clear discussion of the issues and misunderstandings that have arisen in these interdisciplinary conversations. His survey is up-to-date and includes recent developments in philosophy, sociology, anthropology, history, cultural studies, and feminist studies.

By moving from the discipline-bound blinders of a sociology, history, philosophy, or anthropology of science to a transdisciplinary field, science studies, Hess argues, will be able to provide crucial conceptual tools for public discussions about the role of science and technology in a democratic society.

Take a look at Tomorrow's Professor: Preparing for Academic Careers in Science and Engineering

The book is focused on STEM professors, but most of the research and advice is probably cross-discipline. It's probably the best book about what you're in for as a graduate student, postdoc, and tenure-track professor.