As a narrative, Mark Bowden’s “Worm” leaves something to be desired. As a warning, it’s distinctly unnerving.“Worm” is the story of Conficker, a nasty piece of software injected into the Internet’s bloodstream in 2008. Since then it has infected millions of computers worldwide, assembling the largest-ever “botnet” -- a drone army of machines that, unknown to their owners, might be ordered at any time to unleash digital havoc that could bring down global communications systems, power grids, perhaps the very Internet itself.Where did Conficker come from, and who was or is behind it? No one knows, including Bowden, the author of “Black Hawk Down.” Perhaps it’s the Ukrainians, or the Chinese, or a “dark Symantec” -- a commercial criminal enterprise established as the mirror image of the well-known Internet security firm. Whoever they were, though, they clearly knew what they were doing.The worm combined the best-of-breed malevolence from previous malware exploits -- chronicled here in sometimes mind- numbing detail -- with unique advancements of its own. Once it infected a computer, it would repair the Microsoft Windows security breach it had exploited, preventing competitors from coming in behind it. It would also stealthily thwart any effort by the computer’s owner to install a cure. And then, like a terrorist sleeper cell, it would simply await instructions from its unknown master.Combating this mortal threat was an ad hoc group of security experts calling itself the Cabal -- in Bowden’s telling, a sort of Justice League of America of computer nerds desperately racing against time, official indifference and its own internal divisions to head off computergeddon.They included representatives from Microsoft, think-tanks and the loosely organized bodies that pass for the Internet’s governing authorities, as well as assorted other industry consultants.Conspicuously absent was any representative of the U.S. government, despite repeated efforts to engage it. The silence from official quarters proved baffling, until the truth dawned on the Cabal. “The real reason for the feds’ silence was ... they had nothing to offer!” Bowden writes. “They were in way over their heads.”With a deadline of April 1, 2009, when the botnet was programmed to become operational, the Cabal struggled to unpeel Conficker’s protective layers, figure out how it worked and head it off.Every day, the worm would create and check in with long lists of randomly generated Internet domains, behind any one of which its master might lurk. Only as the date approached on which the zombie army would rouse to life did the populace at large become aware of its peril. The New York Times ran articles. “60 Minutes” did a segment. The world held its breath.And then ... nothing. The mighty beast shuddered, sent a little spam and then lapsed back into whatever digital muck spawned it.Or did it? Bowden makes a case that the unknown botmaster’s goal wasn’t to unleash a single crippling attack but to create an infrastructure that can be exploited at will in the future.“The Conficker botnet, this enormous concentration of computer power, had been assembled and was still in the hands of its mysterious creators,” he writes. In hacker jargon, the millions of infected machines remained “pwned, or owned, and they could be turned to any task the botmaster defined. They could be leased for plunder or marshaled for attack.”The author does his best to maintain the story’s dramatic tension. But a combination of factors -- the lengthy history of malware near the beginning of the book, the lack of knowledge on who perpetrated Conficker and why -- all conspire to make “Worm” an oddly unsatisfying yarn.So, too, does a series of annoying factual errors. Personal computers were widely available in 1984, contrary to the book’s assertion. Microsoft’s first product, for the pioneering Altair microcomputer, was a BASIC language interpreter, not an operating system. And so on.As a cautionary tale, though, “Worm” is worth attention. Government officials up to and including President Obama have taken notice of Conficker and begun to address some of the issues it raised. But you’ll probably put “Worm” down with the thought that cyberspace is just as dangerous as the physical world we inhabit.