Overall, an interesting and fun read. The premise is certainly shocking, and paints an incredibly frightening picture of multinational corporate America. And the book is certainly gripping enough; it's actually kinda shocking to find this sort of work over in the nonfiction section.
Perkins certainly isn't going to win any accolades for his writing, though. His style is rather turgid and chock-full of cliche: all the women are shockingly beautiful, the men dark, dashing and mysterious. It's basically the non-fiction version of a bad Grisham knock-off. Perkins seems to see himself as a sort of "international man of mystery," which contradicts his main premise that it's not a Spy-vs-Spy world, but one where the hit men operate openly.
Worse, Perkin's credibility is hard to assess: the people, companies, and agencies mentioned have issued more or less categorical denials — and yet, if the story's true, that's exactly what you'd expect them to do! It's a weird thing, then: the broad outline of Perkin's premise — multinationals serving as proxies for US foreign policy and vice versa — is eminently believable, as is the mechanism by which foreign debt flows into US-based contracting companies. But the details are often too precious to swallow — e.g. being whisked out of Tehran by a childhood friend just ahead of the fall of the Shah, or predicting the rise of the West/Islamist conflict in 1970. At times this preciousness seems almost Forest Gump-ish — "Look! Here I am with Graham Greene!" — and this gives the whole thing an inauthentic air.
It's similarly hard to buy that Perkins was always so torn by his work; I can't really see how you spend a decade doing something you know to be so evil.
I think it's clear, then, that distance and 20/20 hindsight color everything about Perkins' perceptions of his old job. In the end, I believe that Perkins believes what he says, but the details seem clearly colored by some combination of ego, distance, and authorial license.