The Assassin by Clive Cussler and Justin Scott

The Assassin is part of the entertaining Isaac Bell series featuring a turn-of-the-century (20th century, that is) detective. This book is actually set earlier than some of the others, namely in the late 1890s, when John D. Rockefeller and his Standard Oil company were monopolizing the oil industry and putting many smaller operators out of business. One of these operators decides to get revenge on JDR and hires an assassin to systematically destroy the reputation of Standard Oil. Isaac and the Van Doren Agency are on the case, trying to find the assassin before anyone else gets killed.

As always, there is plenty of action and adventure with a little history thrown in.
What I was missing was Isaac's true love, Marion. But I finally realized that he hadn't met her yet.
If you've enjoyed the rest of this series, you will like this one too.