I'm referring to APA citation. Say I'm picking up a train of thought of an author and I know which book he states it in. I just don't have a page number. Is it okay to just say (Muller, 2008) without page number? Will I have to include page numbers for the book in the references?
It depends on the context, of course.
I am a reviewer on an academic journal and there, we ask authors for the page.
I've also been told by a prof that, for a homework essay, he only spot checks a few cites because he "knows the author's voice," (may be more applicable to certain fields than to others) and if the quote is suspect then he will check more thoroughly. If I were you, I'd include the page cite just to be safe. I'm assuming this is for a class - has your prof said anything about how stringent the cites need to be? That would be more determinative than SE's speculation.