While The Wheel of Darkness continues the story of Agent Pendergast, it is a stand alone and not connected to the Diogenes arc. It is a very different story than the previous books, where Pendergast faced his nemesis and brother. Compared to those, this book is calmer, slower, and a bit less exciting. The height of suspense of the previous books isn't reached here. I was nearly midway through before I felt the pace and excitement pick up.
There are far fewer characters as well, though that's not necessarily a bad thing, since some of the characters in the Diogenes series felt a bit extraneous. It also takes place nearly entirely on a ship, and the lack in change of setting sometimes felt limiting.
Pendergast has to deal with some unique difficulties here, though I felt that part of the story could have been explored more than it was. I was glad to see Constance acting more independently, though.
All in all, while an interesting read, and exciting for a good portion of the story, the book felt like a hiatus from Pendergast's usual adventures. Although, considering their location at the start of the story, and what they've already been through, perhaps this was what the authors felt Pendergast and Constance deserved.