I'd heard better things about the film adaptation, however, so I gave that a shot. Let's take a look.
Using the resources of the magazine George/Ray edits, a crime-news magazine with a reputation for solving crimes the cops can't (or at least before the cops do), they put George/Ray in charge of finding this mystery man so they can take care of him.
George/Ray, a very intelligent man who fully realizes what's going on, does his best to obfuscate the situation while trying to find a way to definitively prove Janoth was the murderer without admitting he himself was there (because his wife will find out about his infidelity; in fact, in the novel, he describes it as a choice between doing the right thing and losing his family or saving his skin and losing everything else).
The situation gets tighter and tighter as George/Ray had no reason to cover his own tracks, so it's quite easy for his reporters to him track down (though none ever realize it's him), until he's literally trapped in the magazine's offices and then...
That's where the big difference lies.
In the novel, Earl Janoth initially wants to turn himself in for the killing, but his business partner, Steve Hagen, convinces him that Janoth Publications can't survive without him and he'd be putting thousands of people out of work by turning himself in. At the same time, Janoth has been fighting off a hostile takeover attempt by a rival company. In the end of the novel, just before George is about to be caught and have to make his own hard choice, it's revealed that earlier that day, the rival company's takeover succeeds and Janoth then calls off the investigation, as there is no more purpose to trying to save his own skin, then kills himself. It's a weak ending that actually made me angry as it let George completely off the hook without him ever having to make the really difficult choice that he has struggled with most of the novel.
In the film, Ray finds evidence linking Hagen rather than Janoth to the crime and confronts the two men. Hagen, panicking, points the finger at Janoth, who calmly says he'll stick by his friend and get him the best legal defense available. Hagen, furious, reveals that Janoth killed the girl, whereupon Janoth kills Hagen and tries to flee, only to fall down an elevator shaft to his death. It's a much more exciting ending, though it, too, feels contrived, albeit in a different way.
All told, I do think the film is superior to the novel. There are a number of great shots, and a surprise find that I enjoyed:
At any rate, in this case the film > the book, so skip the novel and watch THE BIG CLOCK, if you're interested (and ignore all the stupid clock references in the film; the film-makers clearly didn't understand the clock metaphor from the novel).