The VTK textbook offers thorough descriptions of important visualization algorithms and techniques that can be found in VTK along with some examples.

More examples and how-to guides can be found at the examples website. Check also this script for getting examples focused around specific classes.

Community discussion takes place on the VTK Discourse forum.

Commercial support and training are available from Kitware.

There is also a collection of technical guides related to VTK that have been published as blog-posts in the past:

For more posts related to VTK see here.