Dark Angel by John Sandford
Letty Davenport's days working a desk job are behind her. Her previous actions at a gunfight in Texas - and her incredible skills with firearms - draw the attention of several branches of the US government, and make her a perfect fit for even more dangerous work.
The Department of Homeland Security tasks her with infiltrating a hacker group that is intent on wreaking havoc nationwide. Letty and her reluctant partner from the NSA pose as free-spirited programmers for hire and embark on a cross country road trip to the group's California headquarters.
But soon they begin to suspect that the hackers are not their only enemy. Someone within their own circle may have betrayed them, and has ulterior motives that place their mission - and their lives - in grave danger.
From one of the world's greatest living mystery writers, Dark Angel is an absolute must read for any thriller lover.