Tips and information that applies to both maps and bookmaps.
When setting up a map or bookmap that will be used to define a book-like deliverable (either as a PDF or online format that breaks topics into sections or chapters), it is common to create a root map with submaps (or chapter-maps). The root map would, at a minimum, contain topicref (or chapter, appendix, etc.) elements that point to each submap (chapter). If your root map is a bookmap, it would also contain the frontmatter and backmatter along with the appropriate child elements. It might also contain a relationship table, but depending on your structure, you might want to maintain relationship tables in the submaps.
The submaps should be standard DITA maps (not a bookmap). If you are using the default DITA-FMx XSLT import script that is provided with the Book application, your chapter maps should contain a single root topicref element which defines the chapter title and any optional content that would appear before the first H1-level heading in the generated FM file. Any topicref elements that are children of the root topicref become the H1, H2, and so on, headings within the chapter.
If you want to be able to use the titles from the submaps as the chapter titles, you’ll need to modify the XSLT import script to pull that content from the map and insert it into the proper location in the output.
This method of creating chapter maps makes it very convenient to assign a “chapter” to a specific writer. Note that it is perfectly reasonable to take this nested map concept to further levels. You may have components within a chapter that make sense to group into a map. This is especially useful if you reuse these components in other maps or deliverables.