I am number four, by Pittacus Lore

There were nine children sent to Earth to escape the destruction of their planet Lorien by the evil Mogadorians. Now the Mogadorians are hunting the nine to kill them all, and three are already dead. Number Four knows they are coming for him next. But at 15, Number Four is sick of moving constantly, changing schools, changing names, never making friends or having a girlfriend. So when he settles into Paradise, Ohio, taking the name of John Smith, he refuses to stay anonymous as he has always done before. He makes friends, falls in love, and when the Mogadorians come for him, he fights. This is good fun, and if it doesn't always make perfect sense, well, that's easy to forgive in the adventure of it all.

Black hole sun, by David MacInnis Gill

Durango is a 16-year-old mercenary on Mars, barely scraping by, when he is offered the job of protecting the treasure found by an an impoverished band of miners from a race of vicious cannibals. The action is fast and violent. Newly colonized Mars is inhospitable and lawless. And Durango is wonderful - tough as nails, as principled as King Arthur, a hero straight out of the Wild West keeping his small band alive by bravery and wits and an unbreakable code of honor.

"Have gun will travel reads the card of a man -- A knight without armor in a savage land -- His fast gun for hire heeds the calling wind -- A soldier of fortune is the man called Paladin -- Paladin, Paladin, where do you roam? -- Paladin, Paladin, far, far from home." - Theme song from Have Gun, Will Travel (1950's TV Western) If they ever make a movie of Black Hole Sun, I want this as the theme song.

Ender's game & Ender's shadow, by Orson Scott Card

I read Ender's shadow several years ago when it was on the Abe list, and enjoyed it a lot. It's taken a while, but I finally got back to the series, and this time read Ender's game, which is the 1st book in the series. These books essentially tell the same story from the perspectives of two different characters, and both are excellent.

Earth has been under alien attack for almost a hundred years, and the next attack is imminent. On earth, humans have genetically engineered children with military genius, and these children are sent to a space station battle school, where they are taught military strategy. Ender Wiggins is the world's best hope to survive, but he is young (six, at the story's opening) and has a lot to learn in a short time. His teachers manipulate behind the scenes, and the older students resent Ender's advancement as Ender struggles to survive the war games that take up most of the battle school curriculum.

Ender's shadow concerns Ender's right hand child commander, Bean.

These are science fiction at it's best - but don't take my word for it. They are on every core collection list I've ever seen and have won numerous awards for Science fiction. Great stuff!