Get the VTK Textbook!
The Visualization Toolkit: An Object-Oriented Approach to 3D Graphics, 4th Edition is the official reference guide for VTK. It provides thorough descriptions of important visualization algorithms, including example images and code. There are two ways of obtaining the latest version of the VTK Textbook.
Printed Edition
The VTK Textbook (ISBN) is offered through various US and international sites.
PDF Download
The VTK Textbook is distributed as a free download in PDF form here. It is released under CC BY 4.0.