This probably sounds unhelpful, but many sources advise against trying to do anything mission-critical with master documents: the effort you put into setting things up again while you move to 2010 could be setting you up for more effort when things go wrong. You may want to trawl this forum for comments before going any farther <g>

Many people also say, more positively, that Word is fine with long documents and doesn't really require you to use a master document mechanism. Try just rounding up all your content into a single [new, separate] Word 2010 file and see if it behaves! book-ish details such as chapter first pages and running headers are usually handled with sections, but you probably knew that already.