His early novels were among his best. In fact, sadly, most of the novels he wrote in the late 1970s and 1980s weren't very good--he'd pretty much lost his mojo late in his career. But when he was on track, he was a magnificent storyteller.
His first book was 1955's HMS Ulysses, an intense tale about a WWII British light cruiser on the Murmansk run, braving horrible weather, intense cold and constant attacks by planes and U-Boats. It is a riveting and (despite a tad too much melodrama in the characterizations) fairly realistic novel, with MacLean drawing on his own wartime experiences for verisimilitude.
But MacLean introduces all sorts of twists and turns into the story. First, to get onto the island, the commandos must scale an unscaleable cliff (at night during a storm, as it turns out). Then they must dodge German search parties after the enemy gets lucky and discovers they are there. They get captured and have to improvise an escape. They get shot at, dive bombed and shelled. But, by golly, they keep going.
In addition to that, there's even a whodunit element involving exactly how the Germans are getting information about the commandos. All of this added on top of the time pressure to get the mission done make the novel truly exciting from start to finish. Guns is in many ways a template for many of MacLean's novels, with the best of them nearly overflowing with unexpected plot twists and surprises.
MacLean's characters are fun to hang out with as well. The leader is Keith Mallory, given the job because he
Their explosives expert is an American named Dusty Miller. MacLean really does overdo Miller's Western drawl, but the character is still likable. And one can argue that the drawl helps mask just how smart and capable Miller is, making some of his actions later in the novel an effective surprise.
As with some of his other early novels, MacLean does pile on the melodrama a little too thickly from time to time, but never enough to spoil a reader's enjoyment. For instance, one of the commandos is a young soldier named Andy Stevens, who is constantly afraid that Mallory and Andrea will realize he is afraid; or that he'll fail them at a key moment. On the one hand, Andy's character arc is cliched and predictable to a degree. On the other hand, MacLean presents Andy in a believable and empathetic manner, so we are sincerely rooting for the guy the whole time.
In the 1961 film, character relationships were changed radically (though the film still follows the basic plot very closely). Mallory and Andrea, for instance, were changed from best friends to deadly enemies--Andrea is planning on killing Mallory once the war is over and they are done killing Germans. It's a complete 180 from their relationship in the book. The film was meant to have more of an anti-war theme, so this change and other character changes were made to highlight this.
The film really is a classic--exciting and visually stunning, but I would argue it doesn't pull off the anti-war theme at all. Francis Truffaut once said that its impossible to make a true anti-war film because movies will invariably make the action seem exciting.
That's particularly true in the film version of The Guns of Navarone. Both the book and the film make strong points about the brutality of war and the necessity of leaders making decisions that will cost lives no matter what. The film adds points about how war can break people emotionally. But in the end, the film succeeds too well in making everything about the mission look awesome. When those guns blow up, we're not thinking "What a horrible experience this must have been!" Instead, we're thinking "Oh, man, that's cool!" and subconsciously wishing we'd been on the mission as well, fighting alongside Mallory and Andrea.
And maybe that's okay. Heck, if real life intruded too far into most adventure stories, then there'd be no sense in telling them. Most of us will never be given the job of blowing up Nazi cannon, which is just as well because most of us would really stink at the job. But there's no reason we can't blow 'em up vicariously through the bravery and cleverness of Mallory, Miller and Andrea--and we can do so via the novel, the movie or even the mind-numbingly cool Marx playset.