Spy books these days are apt to start with an episode that, chronologically speaking, belongs in the middle of the book but that is so enthralling, the reader can’t help but read on to see how it all ends. The writer, his fish hooked, then goes back to the beginning and reels his catch along bit by dramatic bit to the climax.

Joby Warrick makes a pluckier start in “The Triple Agent.” He begins with his climax — a suicide bombing at a CIA base in Afghanistan in 2009 — and trusts in his skill to go back and fascinate readers without the “How does it end?” bait. His trust is well placed. He carries off “The Triple Agent” with admirable drama.

The book’s anti-protagonist is Humam Khalil al-Balawi, a mild-mannered Jordanian doctor by day but a pseudonymous blogger of terroristic screeds by night. So influential was he among jihadists that in early 2009 the Mukhabarat, the Jordanian secret police, went to the trouble of uncovering his identity and, after roughing him up, turned him into a double agent.

Balawi was sent to lawless northwestern Pakistan, where he infiltrated the upper levels of the Pakistani Taliban in an unbelievably short time. At least, it should have been unbelievable. But the Mukhabarat and its bedfellow the CIA were so eager for an inside man that they ignored skeptics who questioned his rapid ascent. After Balawi sent a grainy video of himself with Atiyah Abd al-Rahman, a top aide to Osama bin Laden, he became known in Langley as the “golden source.” How al-Qaeda let such a video be taken should have been another cause for skepticism. So too Balawi’s nearly miraculous claim shortly thereafter that Ayman al-Zawahiri, al-Qaeda’s number two, asked him to be his doctor.

Warrick, a Pulitzer-winning reporter who covers spies for The Post, says Balawi may actually have given some useful information to the CIA — information, for example, that let the agency kill several Taliban via air strikes. But the only definite killing that Warrick reports is one in which the Taliban asked Balawi to prove his ties to the CIA by calling in an air strike on a decoy, a foot soldier posing as a Taliban leader, sacrificed to the cause.

‘The Triple Agent: The al-Qaeda Mole who Infiltrated the CIA’ by Joby Warrick. Doubleday. 244 pp. $26.95. (Doubleday/Doubleday)

Warrick takes a Bridge of San Luis Rey approach to his story, following several characters as they make choices that lead them, almost ineluctably, to what was supposed to be a debriefing of Balawi outside the Afghan city of Khost. Among the protagonists are the Mukhabarat officer who “turned” Balawi, the officer’s CIA counterpart and close friend who doubted Balawi had been turned but went to Khost nonetheless, and the CIA chief in Khost who disregarded concerns about Balawi’s allegiance and let him into the base unsearched so as not to offend him. A lesser presence is then-CIA director Leon Panetta, now secretary of defense, who proved the ultimate overeager believer in Balawi.

Warrick’s account is potent, swift, sometimes prone to the lazy phrase (“eye-popping,” “blowing off steam”), descriptive rather than reflective, anonymous in voice rather than textured and, notwithstanding the gutsy ending-first structure, undaring of prose. He has a skill with words but lacks a felicity with them, and he is not much interested in exploring the large moral questions his tale raises.

He is also firmly establishmentarian. He is loath to call America’s “harsh treatment” of terrorist suspects what it is — beatings at best, torture at worst — and he calls the CIA’s program of extraordinary rendition an “alleged kidnapping and torture” program, though it undeniably is one. He also lapses now and then into stenographic journalism, as when he reprints virtually without question the CIA’s claims that its drone strikes have killed few innocents.

But the benefit of an establishmentarian reporter, if he is good, is that he can get a variety of establishment sources to talk. Warrick is very, very good. He burrows deep inside not only the CIA, which might be expected, but also the Mukhabarat and ISI, Pakistan’s main spy agency. To take just one example, he unearths in some detail how one Taliban faction wanted to kill Balawi on suspicion that he was a CIA spy while another prevailed in protecting and elevating him. Nearly the only important piece of the story Warrick wasn’t able to uncover was how Balawi and his Taliban colleagues settled on the attack at Khost rather than, say, using him to misdirect American air strikes.

However the decision was made, on Dec. 30, 2009, Balawi was whisked past the sentinels at Forward Operating Camp Chapman, whereupon he stepped out of his car, mumbled “La ilaha illa Allah” —“There is no God but God” — and blew up himself and nine CIA and military officers and contractors. With them died the hope for a golden source. At least, Warrick gives no hint of another golden source, which is one reason it took a decade to kill bin Laden and one reason that, despite drone strikes by the score, the Taliban and their terrorist allies flourish still.

Steve Hendricks is a freelance writer whose latest book is “A Kidnapping in Milan: The CIA on Trial.”