The documentation for the C++ interface of Marble is available at api.kde.org.
Further information can be found in TechBase Wiki pages.
The examples directory in Marble's sources has C++ and Qt Quick (QML) examples that give a good idea how to integrate Marble in your project.
A couple of these examples have been converted to TechBase Wiki pages for a more convenient Internet access.
The KML guide informs about using the Keyhole Markup Language in Marble.
The blog series Marble's Secrets gives an insight how some of Marble's features are implemented.
Keep in mind that the Marble's Secrets series was written in 2008 and hence now contains some outdated information.
Information about several topics is also stored in Techbase Wiki pages.