Is it appropriate for a professor to require students to sign a non-disclosure agreement before being taught? There's a professor at my school that claims to be writing a book. He is requiring students to sign a non-disclosure agreement (NDA) before certain lectures because he wants to maintain certain ideas of teaching his content part of his intellectual property.

I think that part is reasonable (to an extent) since he is protecting his ideas.

What I think is unreasonable is the fact that he requires all students to sign these NDAs, and offers no alternative presentation of the subject matter. Thus if a student does not wish to sign, they end up missing out on lectures that they have already paid the university to receive, and they have a potential of missing out on graded content that is "covered" by the material that the professor is restricting. 

Students have no notice that this will happen prior to entering the class and thus are forced to sign if the don't want to withdraw.
No. It’s not appropriate and this is an obvious, blatant abuse of authority. I have a word in mind to describe this professor, but unfortunately am not free to disclose it as I am under an NDA. (It starts with an “i” and ends with a “t”).

Edit: to those asking “why not?”: teaching the class is the professor’s job. He is literally (in the literal sense of “literally”) required to teach the lecture and to allow any registered student who isn’t being disruptive to attend it, without setting any preconditions. And of course this swearing of students to legally binding secrecy is even more absurd than other types of conditions, considering the students are there to acquire knowledge they’ll need to use later in their studies and career. The whole “I’ll teach you but you have to promise not to tell anyone about it” thing reads like something straight out of Catch-22.