The VTK User’s Guide comes complete with example images and code. It is available in print (ISBN) through U.S. and international Amazon sites. It is also distributed as a free PDF download under CC BY 4.0.
The Visualization Toolkit: An Object-Oriented Approach to 3D Graphics, 4th Edition, is the official reference guide for VTK. The textbook describes key visualization algorithms with the help of images and code. It is available in print (ISBN) or as a free PDF download under CC BY 4.0.
The VTK community is working to transcribe and update the textbook. A draft version is available in LaTex or Markdown.
Doxygen documentation for VTK is published nightly as part of VTK’s continuous integration tests. You can refer to these pages to look up the API and purpose of various VTK classes as well as code in the form of linked tests and formal examples.