"The Dark Design" is the third of four books in the main current of the "Riverworld" series. We pick up from where "The Fabulous Riverboat" left off.
A friend of mine liked this installment fairly well, but I think of it more as an arrow in flight than as a standalone work. The bow was strung and pulled taut in "To Your Scattered Bodies Go" and let loose somewhere between the end of "The Fabulous Riverboat" and the beginning of this book. It remains in flight throughout this book, destined to find its target in the final installment ("The Magic Labyrinth"). According to the author, the two books were originally intended to be a single installment, which explains a bit.
A good serial has the initial setup, and a series of smaller mysteries orbiting around the central mystery (or mysteries). Each subplot or portion of the underlying plot is carried forward through the eyes of one or more of the characters. This formula should seem familiar to anyone who has watched "Lost" or any other modern serial.
Thankfully, the Riverworld series doesn't quite attempt to spin one's head around quite as quickly or as frequently. What it does is pull an ensemble cast of characters from throughout history and set them in motion on an epic journey to find out the meaning of the new life they have been granted.
This is a pivotal installation in the series, and stops just short of the final goal. It's a good book in a great series, and well worth the time. Quick tip: pick up "The Magic Labyrinth" at the same time, if you're anything like me, you'll want to dig into it right after finishing this book.