Aragorn is a Dunedain, a superior race of humans who were blessed by the gods (before Sauron caused their downfall). That's why he's the rightful king, and why he has such impressive feats of strength, intelligence and courage.
Where do the books detail that? I've read them, but I never caught that before. I thought the Dunedin were just another ethnicity of men. That's pretty cool though.
It's most throughly detailed in The Silmarillion, but you'll find some scattered references to Númenor in the main trilogy. Essentially, the Númenorians were deceived by Sauron shortly before their end, but some refused to abandon Eru (god), and were blessed
They're outstanding books, if you enjoyed LotR – beware, though, that the Hobbit is a bit more childish and Silmarillion is somewhat disjointed if taken as a single story (it's more of a collection of different stories)