The fifth book in Frank Herbert's classic Dune series maintains the story well.

It's rather fashionable to dismiss Herbert's later books, particularly the later Dune books, as inferior to the originals. But that's unfair. Heretics of Dune clearly shows that Herbert's abilities were not flagging as he carried on the series; the one weakness that the book can be charged with is that it is clearly not complete in itself, but rather obviously a "middle" episode in an ongoing saga.

Heretics has much of the depth of thought, characterization, and dazzling intellectual intricacy of the previous books in the series. Herbert wrestled with profound and thought-provoking concepts. It's our misfortune that he died before being able to finish unfolding his great story.

Speaking of which, if you have any respect for Frank Herbert's works, avoid anything written by his idiot son Brian as you would the plague! In every way, he's the opposite of his brilliant father, as an author; clumsy, stupid, and obvious. Apparently writing ability is not genetically transmitted - either that, or, well, delicacy forbids me from speculating on Brian Herbert's ancestry.