I have multiple experiences from multiple teachers that they hide the book they are secretly following and picking problems from, and whenever students ask for a reference book, they give a multitude of names except for the book which he is secretly following.
As a result, students either waste a lot of time figuring/finding out the answers to the questions or never find an answer which the teacher wants them to write, and consequently most (or some) of them fail. So, those failed students have to repeat the same course multiple times, and others collaboratively build and maintain question banks.
Why do teachers do this?
How ethical is it to hide information from students?