Is it appropriate to ask a professor for a discount on his expensive book? My question is similar to this  - Rude to ask a book chapter.
I have bought the first of two parts of a research monograph. The second part is even more expensive (around $150) and in my currency it is prohibitively expensive. I do not know the professor and he does not know me either. On the other hand it is possible we may collaborate in the future as we are in the same field. Is it appropriate to send him an email and ask him for a discount on his book? Would it harm a professional relationship later on in my career?
Authors usually don't and cannot sell directly copies of their books, and the price is decided by the publisher. 

Of course, an author has their draft copy in electronic format, but they might not even have the final electronic version as published. Authors typically receive a few courtesy copies of their books, but they have no access to other copies for free.

My suggestion however is to borrow the book from your university library: that's what library are for, after all.

If the library doesn't have a copy, depending on your position, you can:


Suggest the library to buy a copy. Many university libraries buy books according to the suggestions of their faculties: if you cannot do this directly, probably your adviser can.
Ask your library if they can loan it from another library. In many countries there is a system of interlibrary loans, and sometimes it works even across different countries.