Summary:
A long series, X/1999 (called simply "X" in Japan) concerns the end of the world as it revolves precariously around a single boy, Kamui, and those who jostle alongside him. Kamui must make a decision: side with the Dragons of Heaven (those who wish to save humanity) or the Dragons of Earth (those who wish to destroy humanity to save the earth). Religious and occult references abound in this somewhat violent piece. Many colorful characters do battle with each other as allegances shift and truths are revealed. X/1999 is among Clamp's most popular series. At this time, Clamp is taking a break from the manga, which will remain unfinished for the foreseeable future. Viz has been publishing the manga in English.
An OAV was created which boiled down the entire story (with an ending) into 2 hours and was generally disliked by fans and non-fans alike.
Recently, a TV series appeared which took its time with the story and is much more satisfying for viewers. This, too, had an ending, though different from the OAV.

Best For:
Ages 16 and up.

Contains:
Drama, Violence, Fantasy, Very Light Shonen-ai (male-male romantic elements)

Read it: Buy the English-translated manga Vol. 1: Prelude, Vol. 2: Overture, Vol. 3: Sonata, Vol. 4: Intermezzo,
Vol. 5: Serenade, Vol. 6: Duet, Vol. 7: Rhapsody, Vol. 8: Crescendo, Vol. 9: Requiem, Vol. 10: Fugue,
Vol. 11: Interlude, Vol. 12: Movement, Vol. 13: Lament, Vol. 14: Concerto, Vol. 15: Waltz, Vol. 16: Nocturne,
Vol. 17: Suite, and the last volume for the time being, Vol. 18: Inversion.

Watch it: The X TV series is out on DVD: X Vol. 1 WITH Box or X Vol. 1 WITHOUT the box.
You can also buy X Vol. 2, X Vol. 3, X Vol. 4, X Vol. 5, X Vol. 6, X Vol. 7, and the last volume, X Vol. 8

Admire it: You can buy the Japanese-language import artbook X - Zero Illustrated Collection.