Do universities maintain secret textbooks? 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.
I have never heard of a textbook being "secret" in the sense that it was being kept intentionally hidden to give students of one organization an advantage.

I have often, however, encountered draft textbooks that a professor had not yet completed or published, for which students were partly acting as "beta testers."  These are often not released for general use (for obvious reasons), and may be in preparation for many years, which could make them act something like a "secret" textbook.

Indeed, as the stage and organization of such documents becomes earlier and less formal, the boundary between "textbook" and "lecture notes" becomes unclear, and there are certainly some professors who are effectively writing a private textbook and just never get around to putting in the effort (or relaxing their perfectionism) to make it public.