The Epic of Gilgamesh is the world's oldest known story. Although our most complete version of it is from Assyria around 700 B.C., fragments of the story have been found from as far back as 2,000-ish B.C. Any writing that we have older than that consists only of records of commercial transactions, plus maybe some poems or prayers. If there are older written narratives than Gilgamesh we don't know of them.
It must have been fairly widely known around the Middle East back at that time because fragments have been found in several languages--for instance, its most famous episode, an account of a flood that destroys all life on earth except one righteous family, of course appears in an only-slightly modified form in the Bible, and was found also in the traditions of many other peoples. However, from the time of antiquity until an archeological expedition uncovered it in the early 20th century the rest of the story was lost.
The story follows Gilgamesh, a young and vigorous king of Uruk. Unable to find a match for his physical prowess among civilized peoples, the Gods send the wild man Enkidu to Uruk, where he engages with Gilgamesh in a wrestling match. After battling to a draw, the two become the best of friends, and set out on adventures together. They defeat a monster in the cedar forest of Lebanon, attracting the attention of the love Goddess Ishtar. She proposed to Gilgamesh, who turns her down. In revenge, she casts a sickness on Enkidu, killing him. Heartbroken after the death of his friend, Gilgamesh sets out for the garden of the Gods to find the secret to immortal life. He arrives and encounters several strange characters, including Utnapishtim, the only mortal man ever to be granted immortality, who relates to him the Flood story. Finally, Gilgamesh finds a kind of undersea plant that grants immortal life, only to lose it again. Upon his arriving again at his home city of Uruk he finally realizes the fate of all men is to die, and the best a man can do is achieve glory for himself while upon the earth.
I've wanted to read this book for a long time and I don't know why I never did before; it's pretty short and easy to read. It's basically an adventure story with larger-than-life heroes and epic journeys and magic artifacts. I would think anybody from high school on with an interest in fantasy or fairy tales would find it interesting, and it's also an intriguing document of life in earliest recorded history. Yet despite its distance from us in time, I find the problems and concerns of Gilgamesh are not so different than those of people today: friendship, romance and spurned love, finding your place in the world, and coming to terms with the inevitability of disease, aging, and misfortune.