David Gerrold warns the reader right up front that this novel is didactic and he's not kidding. There's a great deal of moralizing but it's done with purpose. This isn't the strongest book in the series but it may be the most memorable and I think it's the entry in which the series' key themes are most prominent. One of the most interesting aspects of the War Against the Chtorr novels is they're not simply a rip-roarin' military science fiction yarn about humans battling huge, purple alien worms. The series is also about humanity, about change and adaptation. Before it even begins, a substantial portion of the human race has been wiped out by a series of plagues. Everyone remaining is deeply scarred by those events. Faced with an ecological invasion that seems almost unstoppable, humanity's psychological response becomes as important as any scientific or military reaction to the Chtorran threat and that response is varied. We experience it in this series primarily through the protagonist, Jim McCarthy, who is young, thoroughly messed up and often angry, frustrated and confused. His psychological state is well-established in the first two books, as is his vulnerability, and that's crucial to understanding his behavior in this one.
In A Rage for Revenge, Jim finds himself captured, manipulated and indoctrinated into a cult. The cult represents one of the aforementioned adaptations to the alien Chtorran presence. It's an effort to live in harmony with the Chtorran ecology. Gerrold handles this quite well and the chapters involving the cult are both believable and unsettling. The section that follows is unnerving too and the book deals with difficult and uncomfortable themes (including pedophilia) in ways that some readers will find hard to handle. However, when viewed within the larger psychological context described above, I think it works. In other words, we're supposed to feel uncomfortable.
Much of the aforementioned didacticism involves chapters describing the Mode training, another (and controversial) human response to the invasion, designed to help humanity successfully hold itself together and find a winning strategy.
None of these books are for the timid or easily offended. In addition to the controversial themes mentioned above, this one features dirty limericks at the beginning of every chapter (for reasons explained in the book).
I've read this novel 3 times now and although I don't think it's one of the best books I've ever read, it's certainly among the most memorable. As good as it is in some places, it's didactic nature wears a little and there's an aspect of the story near the end that doesn't quite ring true to me. Nevertheless, I recommend it.