Read: Arctic Drift
Within a period of 25 years there is remarkable little change in Clive Cussler's Dirk Pitt series of novels. There is always the same, predictable structure and basic plot: A successful, entertaining recipe. In the more recent novels, next the the historic introduction, environmental issues have become more prominently featured. Arctic Drift, however, very clearly shows a significant change that began a couple of novels earlier.
Dirk Pitt, Sr. is getting old, tired, and hurt. He is not the (only) larger than live hero who saves the day any more. He is not the one who makes sure that the evil (greedy capitalist turned) villain is not going to hurt anyone any more. While still in the field much of the action that in earlier novels defined his character is now happening without him.
This change entails that other characters get more space. Yet, these “new” character are substantially less well development. There is a considerably smaller emotional attachment, less excitement. The Pitt series may slowly (or pretty fast) loose its appeal if this change continues.