The Abyss by Alex Croft

Former clandestine operative Carmen Foxdale is on the mend after being rescued from a dank subterranean Alcatraz Island hellhole. Weeks of brutal torture while being held captive by Latvian mobsters had taken their toll on both her body and mind. After her rescue, she seeks answers as to why she was subject to a kill order and had been disavowed by the CIA. Left with nothing to lose and at the mercy of an ex-special forces operative—who has been tasked with keeping her alive and out of trouble—Foxdale puts her rage to work and begins plotting her revenge.