A paper is published in a book, conference and its proceedings, should I cite it as book, conference or conference proceeding paper?
If you are writing a journal article, follow the journal's instructions.  If there are no instructions, imitate papers they have recently published.
If you are doing homework or writing a thesis, ask your professor.  Do not be surprised if they do not care.
So long as the paper is easy to find from the citation, any of these approaches is reasonable.