Sorry for the delay; my computer was in the shop for a few days getting fixed.
Edit -> These will have certain spoilers, I'm sure, so read with caution (as I'm not spoiler-tagging anything out)
Okay, here's a quick synopsis of Holic.
Spelled xxxHolic, but pronounced as Holic, this series started roughly the same time as Tsubasa and became CLAMP's 4th series at the time (the unfinished X series, Legal Drug, and Tsubasa being the others),
It features a high school student named Watanuki Kimihiro (whos' name in kanji is April 1st, an important piece of information), who has had a unique ability throughout his life; he can see "spirits". Not ghosts, per say, though that does happen from time to time. Watanuki lives alone in a renter's building and one day mistakenly, steps into "Yuuko's shop". Though he tries to leave, Yuuko realizes that his coming here was 'hitsuzen" and enlists Watanuki's help around the shop. After meeting the black Mokona (Larg, I believe is its' name, the white Mokona is Soel, although in the manga, both are just called Mokona) Maru"dashi" and Moro"dashi", Watanuki begins to find that his life is "changing" as in exchange for his services to Yuuko, the witch will remove his ability to see the creatures in his blood. (His blood supposidely causes it, no more explanation has been given as of yet)
Watanuki also gains many friends, his 2 closest being Himawari Kunogi and Shizuka Dōmeki (also spelled Doumeki) from his school. Himawari is the girl that Watanuki seems to long for (who we later find out inadvertly grants bad things to all but her parents), while Doumeki is a boy from a local temple who seems to have the ability to lessen the effect of the spirits that cling to Watanuki. Throughout the stories, these 3 become very close and through many misadventures, become almost blood-siblings in what they've given up and taken for each other.
For example, after Watanuki falls from a window at school, Himawari takes part of the cost so that Watanuki would recover from his injuries. While she takes the scars that Watanuki would have recieved, Doumeki pays in blood lost, while a still unknown 3rd party (though we are to believe that it is the "real Syaoran" from Tsubasa) paid another cost. Doumeki also pays a price of 1/2 his eye's sight to Yuuko in order to let Watanuki not to go blind in one eye (after he loses it to the spider spirit to save the life of another spirit).
In the latter half of the storyline, Watanuki begins to spend much of his time sleeping and finds that he can visit the dreams of others (much like Kakyou from "X" who we meet in Tsubasa:Tokyo Revelations, Tomoyo from Tsubasa (the Nihon arc)). He, in turn, meets with Sakura (in the dreamworld), Syaoran, Doumeki's grandfather Haruka, and others as he slumbers. He then comes to a startling assumption; he believes that he may only exist within the world of dreams! While he is a wonderful cook, he can't remember eating any of his creations. His parents died, yet he doesn't remember their names. He goes to school, yet he doesn't even know what year he's in!
Yuuko, within the dream, tells him that he is indeed human and that someone dear to him (Syaoran, for some still unknown reason) has paid in advance to make sure Watanuki doesn't disappear from the world. Watanuki, then so grateful, asks if there is a wish that he can grant for Yuuko. The storyline has continued from there, interlocking with Tsubasa to form a larger storyline.
While we are getting closer to the end, Holic and Tsubasa have become even more linked through the volumes. CLAMP keeps hinting that more than 1 character may end up with a sad fate and as we rush towards the final chapitres, it makes you wonder just exactly how everyone is connected...
Yeah, I'm sure that was a lousy summary, but you really should read the chapters, they are quite wonderful and are another great story from CLAMP.
Also...I forgot to mention one thing...remember when I mentioned that Watanuki's birthday was April 1st? Sakura and Syaoran from Tsubasa also share the same birthday...