This theme is totally canon all the way up to the end of Hunters of Dune, taking in every book written by Frank Herbert then later Brian Herbert and Kevin J. Anderson. Totally canon to the theme, breaking off at the end of Hunters of Dune, where Duncan Idaho and Miles Teg together ignite another fold-space jump to evade deadly enemies… only to find themselves in the Multiverse. The theme itself has moved on, finishing to the end of Sandworms of Dune, after Duncan and the others decided to find and return to their Galaxy and face the Enemy that waited.
The theme itself is a multi-world space and political opera of immense proportion, full of intrigue and guile, as well as intense moral and philosophical debate. It centers around the planet known as Dune, a desert world where a species known as the Sandworm has come to exist. From the life cycle of this strange species the 'Spice', Melange, is created.
This 'Spice' allows for expanding both human life and human consciousness, and when given enough it has mutagenic and long term effects. It can be used for many purposes, including transforming a human into a prescient warped human known as a Navigator, who is needed to guide ships through Fold-space. It is by the Spice that all space travel, intergalactic trade, and even the power of life and death is made possible. Humanity itself has become addicted to the substance more deeply then it ever was to oil.
The Dune theme is about the control of Spice, control of Dune, and the effects and ramifications this substance has on the whole of humanity.
Whoever controls the Spice controls the Universe.
The Spice must Flow.