Under Fire by Tom Clancy

On a mission in Tehran, Jack Ryan, Jr., meets his oldest friend, Seth Gregory. As they part, Seth slips Jack a key, along with a perplexing message. The next day Jack is summoned to an apartment where two men claim Seth has disappeared with funds for a vital intelligence operation. They say he’s turned and leave Jack with a warning:  If you hear from Seth, call us. Do not get involved.
 
Jack soon finds himself lost in a maze of intrigue, lies, and betrayal where no one is who they seem to be—not even Seth, who’s harboring a secret that harkens back to the Cold War. A secret that is driving him to the brink of treason....