There is an incident towards the end of Heretics of Dune which neatly illustrates why this is the finest book in the Dune series since the original. Perhaps stung by the criticisms levelled at God Emperor of Dune's focus on self-important philosophising, there are several mentions in Heretics' early chapters of explosions and action, as if to promise more conventional sci-fi thrills. What we actually get are occasional bursts of violence punctuating the usual lengthy sections focused on scheming, strategising and portentous sociology. However, in the run-up to the book's conclusion, one of the main characters reaches a heightened level of physical and mental ability and embarks on an orgy of destruction, a whirling kung-fu dervish pulverising everything in its path. Lesser sci-fi books would take the opportunity to indulge in some action porn, detailing every severed limb and hand-chop. Herbert instead chooses to focus on his protagonist's heightened awareness, and the character in question reminisces on childhood experiences and considers the forces at work in the universe whilst distractedly lopping off the bad guys' heads with the occasional sweep of a hand. It's a lovely piece of sleight-at-hand - simultaneously acceding to demands for more action, whilst relegating it to an afterthought. It's an integration of crowd-pleasing and authorial integrity that Heretics of Dune consistently nails, far more so than any of Herbert's previous books at least as far back as the original Dune.
Heretics takes place several thousand years after the events of God Emperor, offering Herbert the chance to clear the decks. Prescience, a subject Herbert had all but exhausted as far back as Dune Messiah, takes a back seat here. Much of the action takes place away from the desert planet itself. The Atreides are still about, but have been absorbed into the Bene Gesserit Sisterhood, on which this book focuses. Melange remains theoretically important, but is of little practical importance to the story. Freed from the usual baggage, Herbert does something he hasn't really done in this series since Dune itself, and creates a batch of new, interesting characters, placed in a cohesive multi-strand plot that works steadily towards a definite end-game.
The final revelation of the Sisterhood's `grand design' isn't particularly convincing, but even this is put to good use by Herbert: the dependency of people on the perceived authority and vision of leaders is a key theme here, one that is gradually expanded to include the question of free will in a universe that may or may not be on a predetermined path set by God (or Leto II, in this case.) In this context, the general confusion amongst characters as to whether anyone really knows what is going on makes perfect sense.
An effort is made to develop characters properly - admittedly limited to a handful of childhood details, the occasional noting of a character's personal preferences, and the odd musing on the nature of love, but this is uncharted ground for the Dune series, where characters are usually far too busy planning the future of humanity to darn their own socks. There's a chapter where one of our grand protagonists goes for a wander through a city full of normal people doing non-epic things - a class of people conspicuously absent from the Dune series up to this point. Even sex makes an appearance (albeit epic, emotionless sex.) This newfound, tentative interest in the muck and brass of humanity ties with another key theme, one present throughout the series but brought into stark relief here - what is it to be human? Is it enough to simply self-perpetuate, or should we have a "noble purpose"? It also gives a welcome freshness to Heretics of Dune, a feel of expanded horizons and new ideas being explored.
It's not perfect - there are longueurs, particularly in the first half, and there is occasionally a sense that Herbert is recycling material. Also, it's more likely than not to be incomprehensible if you haven't plodded through Messiah, Children and God Emperor. Finally, though it is reasonably self-contained with a definite ending, it's also clearly written to lead straight into Chapter House Dune. Still, this is easily the best Dune book since the original, and is in many ways a more subtle and interesting book than `Dune.' It lacks the original's extraordinary novelty and its mythic, archetypal plot, but this is a thoughtful, intelligent and well-structured work that stands with Herbert's best.