Dead or Alive by Patricia Wentworth

Robin O'Hara never deserved his wife, Meg. He was unfaithful and involved in work that was just a little bit dangerous. When Meg reaches her limits and wants a divorce, Robin disappears and is presumed dead. Shortly after, Meg starts receiving messages that gives her the idea that Robin may not be dead after all. When former beau, Bill Coverdale, comes back to London he is intent on winning Meg for himself. He just has to deal with her fears about her missing husband, a crazy group of kidnappers, and save her from having her body dumped into a remote lake, never to be seen again.