The most notable class is Akonadi::ETMCalendar, a model built on top of the Akonadi::EntityTreeModel which provides filters to only show events from selected calendars, iterate over events, including recurrences, provides reverse lookup from KCalCore::Incidence to Akonadi::Item and other features.
Besides the model there is for example Akonadi::FreeBusyProviderBase, an interface for Akonadi Resources that can provide Free/Busy information.