Well. First you should know that there are several editions, with more or less different rules. The most popular is 3.5 edition. But there is Pathfinder, another Pen & Paper game, which is basically a more cleaned up, easier to get into refined version of 3.5 edition. Either way, there are three basic books, Gamemaster Guide-book(s) and Player guide book(s) and the Monster Manual/books. I say book(s), because people always come up with more ideas to add to the game and thus the Player's Handbook 3 might contain new player classes, like ninjas, samurais and killer robots.
People saying "D&D is vast" is an accurate description of the P&P as well, because the Gamemaster makes the game in the image that s/he has and then the players play it. It can be more or less rule-heavy, include more dice-management or be the opposite.
4th edition of D&D, which is the newest published version of D&D, is a lot more like a strategy game, with a very big emphasis on combat, them lasting for even hours.
You might want to look around at other Pen & Paper games as well, to find the type you like. There is GURPS, Generic Universal Roleplaying System, which has a _lot_ of optional rules to play around with, but it allows you to run basically any kind of game you want and in any setting, magic, high-tech, futuristic, fantasy...
New World of Darkness, a new version of World of Darkness, focusing on horror and investigation, with different books to play vampires, werewolves, monster hunters et cetera.