I finally saw the movie, and it was very different from the book. Actually, I listened to the audiobook, partly because it was read by David Tennant. Tennant reads it in his native Scottish accent, which is pretty entertaining.
***SPOILERS***
The biggest difference between the book and the movie is the dragons.
In the movie, the Vikings are at war with the dragons, which are large, dangerous animals. The children engage in "Dragon Training," which is training in how to fight/kill dragons. As part of this generations-long war, some Vikings are looking for the nest of the dragons, in order to drive them away.
In the book, dragons are generally smaller animals, the size of dogs. There is no war against the dragons. Instead, it is the custom of the Vikings to capture a young dragon, and raise it and train it to help the Vikings with their hunting. In fact, it is a rite of passage for the Viking kids to sneak into the nearby nest of dragons during the winter when they are hibernating, and capture their own dragon. The Viking kids must then train the dragon to obey simple orders. Any Viking kid who fails to do so is exiled from the tribe.
In the movie, the main character is Hiccup, son of the chief, who is nerdy and bad at fighting. But he's also really smart, as evidenced by his brilliant inventions.
In the book, Hiccup is largely the same, though he doesn't create any amazing inventions. As the son of the chief, Hiccup has great expectations on his shoulders. As leadership is hereditary in this tribe, Hiccup must grow to be a great hunter and leader to his people. But this seems unlikely.
In the movie, Hiccup has a love interest, Astrid. The other Viking kids include Snotlout, the more Viking-like kid who looks kind of like Jack Black, Tuffnut and Ruffnut, the constantly fighting twins, and Fishlegs, the big, fat nerd who has memorized the stats of the various dragon breeds.
In the book, there is no Astrid. Hiccup is only 10 in the book, so the author may have deemed a love interest inappropriate. Snotlout is the de facto leader of the Viking kids. He is strong, athletic, and basically the high-school-jock version of the Viking kids. He bullies Hiccup, and resents him for being in line to inherit leadership of the tribe. Secretly, he plans to someday murder Hiccup and take over the tribe. Snotlout's sidekick is the big, stupid Dogsbreath. Fishlegs is the small, dorky kid who is Hiccup's best friend, though Hiccup is the one who is compiling the dragon stats.
The dragons in the book actually speak, though they have their own language. Hiccup is the only one who has bothered to learn the dragon language, as the rest of the Vikings are able to train their dragons simply by yelling at them.
In the movie, Hiccup's dragon is a rare and exotic Night Fury, which no one has ever seen before. He is called "Toothless" because he initially appears to have no teeth, though Toothless actually has a full complement of teeth which are simply retractable. While standoffish at first, Toothless quickly becomes an affectionate and loyal friend to Hiccup.
In the book, the dragon Hiccup captures is a small "common or garden" variety of dragon. He's smaller than the dragons the other kids have. He's called "Toothless" because he actually has no teeth. At one point, Toothless does grow his first tooth, but he later looses this tooth while biting another dragon. (In contrast, the dragon captured by Snotlout is of a variety so awesome that only the tribe's chief is supposed to own one.) Toothless is selfish, disobedient, and sassy. He refuses to obey Hiccup's commands, except when Hiccup promises to tell him jokes in exchange for his obedience. Even then, his obedience is grudging and half-hearted.
In the movie, there is a book on dragon training, which describes all of the dragon breeds, concluding "kill on sight" with each of them.
In the book, there is a book called "How To Train Your Dragon" which contains one chapter, one page, one sentence: "Yell at it; the louder the better." The Viking way is to dominate their dragon through sheer force of personality. Unfortunately, Hiccup's personality has no force, so he resorts to pampering Toothless and bribing him with the promise of jokes, which Toothless loves.
The movie climaxes with a Viking attack upon the dragon nest, rousing a giant dragon which is only beaten when Hiccup rides to the rescue with his classmates, whom he has taught to train dragons of their own. Hiccup rides Toothless in a battle with the giant dragon, ultimately defeating it at the cost of his own leg.
The book climaxes with the awakening of a couple of ancient sea dragons who had been hibernating for centuries on the sea floor. Large enough to swallow entire ships, the sea dragons are entirely unaffected when the Vikings attempt their usual dragon-taming tactic of yelling, even when the entire tribe tries yelling in unison. Knowing they have no chance against dragons of this size, the adults are left stymied. Hiccup uses his knowledge of the dragon language and his cleverness to manipulate the dragons into killing each other, with the help of the other Viking kids and their dragons. When the remaining sea dragon tries to swallow Hiccup, the selfish Toothless uncharacteristically comes to his rescue by flying up the sea dragon's nose and making it sneeze Hiccup out. While inside the sea dragon's throat, Hiccup had cleverly blocked the sea dragon's flame gland with his helmet. So when the sea dragon finally tries to kill Hiccup, it explodes.
The movie and the book share the general story arc of the nerdy, smart kid who is ill-equipped to excel by the standards of the Viking tribe, but who saves the tribe through his intelligence and unconventional thinking. But where in the movie, Hiccup becomes awesome by lucking into having the most awesome dragon ever, in the book, Hiccup become a hero despite having the least impressive, least cooperative, most obstinate dragon in the tribe.
It seems like nearly every important story element in the book was altered for the movie. But judging from the success of the movie, I guess it worked.