Edition: New English Library, 1985
Chapterhouse Dune carries on almost immediately where Heretics of Dune left off. It is set almost entirely on Chapter House, the planet which houses the secret headquarters of the Bene Gesserit, who are being hunted to extinction by the Honoured Matres returning from the Scattering, the massive colonisation effort of humanity following the death of Emperor Leto. It is the last Dune novel written by Herbert before his death, though since then there have consistently been rumours of more Dune material which is still unpublished - and Chapterhouse Dune leaves several loose threads which could easily provide the basis for the saga to continue.
In fact, Chapterhouse Dune seems to be intended to imply that at least one further instalment was planned. It cannot be said to be 'climactic' and leaves a great deal unexplained. I would still say that the rest of the series is relatively disappointing compared to the first one, but this is really due to the fact that there is no apparent overall planning which would unify the series. The way that this novel points seems to show that Herbert had come up with a plan, of which Heretics of Dune and this novel form the start.
The main character in Chapterhouse Dune is Darwi Odrade, now the head (Mother Superior) of the Bene Gesserit. She is not terribly well differentiated from the other sisters, with the exception of three others who are rather far from the norm (Bellonda, irascible archivist; Sheeana, the former waif from Arrakis who can control the sandworms; and Murbella, the former Honoured Matre). The most interesting characters are, however, the Jewish rabbi and Rebecca, portrayed as survivors of a very ancient people with many centuries' experience in evading those who wish to destroy them. Rachel has made herself a "wild" Reverend Mother, so that she possesses the abilities of the Bene Gesserit; she now stores within her own consciousness the memories of a billion Bene Gesserit whose planet was destroyed by the Honoured Matres. The idea that the Jewish faith survived unchanged and underground for so many centuries is quite a fascinating one, and is one of the aspects of the book that I suspect would have been developed if the series had continued.