Without much info about the “complexity” of the document, here are simple suggestions.
As @EarnestAl notes, an 800-page document is a good candidate for dividing it into separate files. Your document has certainly an intrinsic structure to make it usable, at least chapters. This would allow to decrease the volume of each component on the order of 100 pages (from personal experience, there is no performance issue up to ~400 pages).
As long as you have no cross-reference across these parts, this is fully transparent.
You then need a master document for the front cover, dedication, TOC, index and back cover. Between TOC and index, you insert “links” to the chapters. This is only a suggestion; adapt the real composition of the master to your case (short chapters could be directly typed in the master but this would distort the “symmetry” of the chapters).
With many files supposed to have the same look, you must base them on the same template to guarantee formatting consistency. A template is a skeleton file containing the needed styles and optionally initial content.
Such an organization also has a consequence on the work flow. Again to guarantee consistency, you must ban absolutely any direct formatting and exclusively use styles in all categories: paragraph, character, page, frame and list. The most neglected category is character because there is nothing equivalent in Word and keyboard shortcuts (Ctrl+B or I) or toolbar buttons are so easy to use. The same goes for lists and picture inserts which should be controlled by paragraph styles associated with a list style and frame styles respectively.
Should you need a style modification, refrain from doing it in one of the documents: this would not transfer to others. Tune the style in the template, then reopen the document your were working with. Styles will be updated. When you open the other documents, they’ll be updated too.
This also has the side advantage that formatting only with styles reduces the internal file complexity and its size.
This may look as a nuisance, make your work troublesome, harder or slower. However you’ll quickly notice this is more systematic and methodical. Remember that styles primarily don’t describe typographical attributes (this is only the end effect). They markup your text for its semantic value. Taking the example of paragraphs only, paragraph styles split your text into headings, discourse, comments, notes. What you do with this classification (font size, weight, spacing) is only a consequence of the significance and translates author intent.
To show the community your question has been answered, click the ✓ next to the correct answer, and “upvote” by clicking on the ^ arrow of any helpful answers. These are the mechanisms for communicating the quality of the Q&A on this site. Thanks!
