Well, what did I think? Read on...
At any rate, one of the first things that struck me is how much this reads like unformed, or perhaps not yet matured would be a better phrase, Westlake. Of course, the film and the novelization predate the first of Donald Westlake's comedy-crime novels by several years, but I do wonder if there was any influence on Westlake as the style really is quite similar.
So, the novel. It's good, but not amazing. It's amusing, but except for a few passages, not particularly memorable. The characters are mostly defined by unusual speech patterns and, in the case of Danny Ocean, Jimmy Foster, and Sam Harmon, by how others refer to them: everyone is or wants to be Danny's friend, so they're very hail-fellow-well-met; Jimmy Foster is either admired or reviled as the millionaire's son; Sam Harmon is simply "the pianist" to many people and "Danny's best friend" to others. One of the best-developed characters is, ironically, the character who's supposed to have the least in his head: Mushy O'Conner. Supposedly, he's a simple thug, but the authors give us a fair view of his real character, and fairly subtly, as he's torn between loyalty to his employer (the gangster Spyros Acedos, who has also employed Ocean) and Ocean, who he feels tremendous friendship and loyalty to.
The first thing that surprised me, and I honestly can't remember how it was in the movie, is how little is spent on the actual crime. Barely a chapter or two is allotted to it while we mostly spend on our time on the various relationships between fourteen or so people. And it was fine, it wasn't what I expected, but it was mostly enjoyable.
The other was how dark the ending of the novel gets. In fact, the last chapter almost seems like it came from another work, its tone is so bleak. But it was also a fine ending, one that the story, and especially Danny Ocean, earned, so I have no complaints.
You probably aren't likely to run across this book, as it's sixty years old and not particularly collectible or anything, but if you do, by all means, read it for yourself. Compare it to the movie and see how it stacks up. As I said, it may not be anything special, but it is enjoyable.