Textbooks are highly important resources for course work as well as for research work.
Some textbooks are freely available over internet. Some textbooks can be purchased.
Is it true that there exists secret textbooks available only for the students of that particular university and for no others?
I had this doubt because one of my batch mates claimed that there is a university (say X) has its secret textbook for students as well as researchers of that university only and it is a common practice for many top end universities.
Note that I am asking about textbooks only, not regarding teaching material and others.