Ideally, a chapter map references topics, and a book map references chapter maps. However, not all DITA tools allow nested maps. Therefore the default DITA2Go option is to have the book map reference topics directly rather than reference the chapter maps.
To include in the book .ditamap references to chapter maps instead of direct references to each topic:
[DITAOptions]
; MapBookTopics = Yes (default, include <topicref> for each topic
in
; book .ditamap), or No reference the chapter maps instead)
MapBookTopics = No
Referencing chapter maps is better practice, if your downstream tools support this approach.