Foreign Agent by Brad Thor

Ex-Navy Seal Scott Horvath has been working undercover for a private intelligence agency, saving the world at the expense of his personal life. But after a series of terrorist attacks, he discovers that he may have met his match in the form of a Russian agent named Sacha. Soon Scott realizes that there is more than meets the eye going on with the terrorist attacks and has to rush to prevent disaster and outwit Sacha. It's quite the nail biter.

Foreign Agent is part of a series, so if you're new to this author, you might want to start at the beginning with Lions of Lucerne.