It's a month full of CLAMP titles! But when the mangaka (the Japanese term for a manga artist/writer) group has been creating series for over fifteen years, maybe it's surprising that it's only a month.
I mentioned Tsubasa, or TRC (not to be confused with Tsubasa: Those Who Have Wings, a series produced by Natsuki Takaya, the mangaka of Fruits Basket) before when talking about xxxHolic; as I said there, the two series occur within the same universe and often reference each other. TRC differs slightly from xxxHolic in terms of story-telling--it was published in the Japanese Shonen Jump, and is a part of the shounen (boy's stories) subgroup of manga, meaning it is similar to other shounen titles like Bleach, Naruto, and One Piece.
Tsubasa Reservoir Chronicles is a fantasy series set in no particular world; in fact, its plot involves four people traveling together to different worlds for their own reasons. Sakura is a princess whose memories were changed into feathers ('tsubasa' is Japanese for wing or feather) and scattered among many worlds as the result of an evil spell; she and Syaoran, a young man from her world, are traveling in order to find and return these memories to her. Kurogane is a ninja who was sent away from his world by the princess he served for being too bloodthirsty; he's traveling in order to return home. And the fourth person, Fai, is a wizard who refuses to use magic and who is traveling because he is running away from something--or someone.
Many of the worlds and supporting characters of TRC are similar to those of previous CLAMP titles. In fact, Sakura and Syaoran are both characters from the series Cardcaptor Sakura, and many of the events in that series have an impact later on in the story of TRC. No world is ever quite like its original story's; but because so much of Tsubasa Reservior Chronicles either relies upon or is improved by a knowledge of CLAMP's previous works, it can make the story a little harder to dive into for those unfamiliar with them. And like xxxHolic, the translation of TRC maintains honorifics (those name-endings in Japanese that are similar to English "Mr." and "Mrs.") and some Japanese words. But also like xxxHolic, each volume has notes on terms and references to Japanese customs.
Tsubasa Reservoir Chronicles is heavily dependent on a prior knowledge of CLAMP's works to understand a lot of its background; but the foreground--the plot and the growth of the characters as they travel, work together, change and sometimes even betray each other--does not require it, so you can read and enjoy the story for what it is: the tale of a group of people from vastly different worlds thrown together for a common goal and learning how to work together and to accept the personal changes that come as a consequence of growing close to other human beings.