In the print dialog box, do you have an Options button in the lower left? Look there for ‘Options for Duplex Printing.’ You may have the wrong order checked. I have an older version of Word, so yours might have this somewhere else. I might also have suggested limiting your file size, perhaps chapter by chapter, rather than having the whole book in one file. I sure hope you back up frequently to another location. Not that Word can’t handle big files…I just wouldn’t be that trusting with my magnum opus.