Dungeons and Dragons three Bibles.

Alright, yes it is a bit over the top to call them the three Bibles but they are the bread and butter of true Dungeons and Dragons. There are three in all that you need to understand Dungeons and Dragons, because it contains everything you need to know! They are also known as core books. Below are the 4th edition, the latest edition of D&D! There are other optional books that provide additional information (such as classes, race and equipment) but these are to boost your dungeons and dragons experience. Such related items can be found in Addition D&D Products.

Version 4.0

Player's Handbook

This book MUST be bought by everyone who wants to play Dungeons and Dragons. It contains information about races, classes, skills, attributes, feats, abilities, spells and equipment. It also helps you create your first character from scratch with a step by step process, telling you what dice to throw and when. Can easily be used as a reference guide to look up spells and skills quickly. Everyone should at least glance through this book before/after playing a game.

Dungeon Master's Guide

This is must have if you want to be a Dungeon Master to write adventures and stories. It gives you hints and tips about writing a good adventure, includes treasure tables so you can randomally assign treasure. Also provides prestiage classes ("advanced" classes) which is explained as well. Includes how to write up encounters as well. However, to get the full use of the dungeon master's guide you must have accompaning it the Monster Manual. Not necessary for players, but necessary for DMs.

Monster Manual

This isn't really good on it's own, it is best to be read in conjunction with the Dungeon Master's Guide. Includes alot of detail about the different sorts of monsters players can encounter. Also includes details about creating your own monsters which can be used in your encounters.

If you're the sort of person who enjoys playing as the character in a story, then just go for the players manual. If you enjoy creative writing then buy all three books and have a read, maybe write some stories up yourself to play through with your friends. Yes, they can be expensive so you could get a group of you to all contribute towards the three books and you can explore the dungeons and dragons world together.
So if you want to write stories, head over to the For Dungeon Masters for tips about how to write up adventures and playing the session.

If you enjoy just playing as a character in a story, head over to For players on tips for character creation, writing a unique character and some role playing tips.