The user manual introduces you to the main elements of the Marble Desktop application (KDE and Qt version). You can open it from within Marble using the F1 shortcut (and from the Help menu). You can also access it online at docs.kde.org.
Does the user manual not answer your question? The following resources may be helpful as well.
You can also get in touch with other users and Marble developers.
Nokia N900, N950, N9
Learn how to make full use of Marble's offline routing and turn-by-turn navigation capabilities on your smartphone with the tutorials from the UserBase Wiki.
We have a support thread for both the Maemo (N900) as well as the MeeGo (N9/N950) version at maemo.org Talk. Both threads provide additional information about the mobile versions.
Our documentation for developers covers using the Marble library in your Qt or Qt Quick (QML) based projects as well as extending Marble using plugins.
Create your own maps! Information about Marble's map format (.dgml), how to create historic maps and possible sources for maps.
During turn-by-turn navigation you can have a speaker announce turns. Learn how to use your own voice here.