I sub-divide things and then put the whole thing together into one Word document as part of the editing process.
You're just making a manuscript, right? Layout and design will be taken care of by your publisher. If you're intending to do the layout and design yourself, you need to contact your publisher to find out how they would want the document prepared.
I'm doing all of the layout/formatting on my thesis, which I'm doing by sub-section (generally, chapters) in individual Word documents. For an article I wrote to appear in a magazine, I submitted the text as a .txt file with no formatting, with scanned and generated images as seperate files--the publisher took care of formatting and layout.
And, yes, as a general rule, you want to set daily goals. Determine that you are going to write 2 solid pages (or whatever), and then do it, even if it is two pages of absolute crap. Crap can be edited, blank paper can't.
Jim