User documentation
A good starting point is the documentation page on the wiki.
The wiki also contains a fairly up to date list of plugins.
Developer documentation
Whitepapers
Universal Relational Storage for Geometric Primitives provides a detailed discussion of how K-3D flexibly stores geometric data, and what this flexibility means for researchers and developers.
Overviews
The K-3D Developers' Wiki covers a broad range of how-tos, design discussions, and architectural information.
Reference
The C++ Reference Documentation covers namespaces, classes, methods, and members, and is automatically generated using Doxygen.
The Python Reference Documentation is automatically generated using Epydoc.