This picture is actually from the anime, but it was a good one to show all the characters. Top (l-r): Sakura and Syaoran. Bottom (l-r): Fei Wong Reed, Xing Huo, Soma, Princess Tomoyo, Kurogane, Fai, High Priest Yukito, King Toya, Mokona (the white thing), Watanuki, and Yuko (the Time-Space Witch). (Picture from Tsubasa official website)
This is a series of manga, which is still being written. Currently, there are 19 books in the series, which has also been made into an anime.
Tsubasa (which means wings in Japanese) follows the story of Syaoran, an archaeologist, and Sakura, a princess, two friends (both in love with each other, though neither knows the other's feelings) from the land of Clow, ruled by Sakura's brother, Toya. When Sakura is affected by some ancient power from the ruins where Syaoran is working, her memories are scattered to different dimensions, each in the shape of a feather. With out the feathers, Sakura will die, so the high priest of the kingdom sends Sakura and Syaoran to the Space-Time Witch, Yuko, who, for the right price, will help them gain the power to travel dimensions. While there, Syaoran meets Kurogane, a ninja, and Fai, a magician, both coming for the same reason as Syaoran: to travel dimensions. However, to do this they all have to give up the most important thing to them. Kurogane gives up his sword; Fai, his magic; and Syaoran has to give up his relationship with Sakura: even if they manage to recover all her memories, she'll never remember Syaoran. For this price, Yuko gives them Mokona, a strange creature with the power to travel dimensions. The story tells about all of the different worlds they visit, and the struggles they have in getting Sakura's feathers back.
Sakura and Syaoran are both crossover characters from Cardcaptor Sakura, which I have read before now. I actually read the first book of Tsubasa on the bus on the way home from middle school, but didn't go any farther than that. After I had read Cardcaptor Sakura, I decided to try this one, since it had the same characters. I have only read up to volume 7 so far, but I'm planning to keep going.
Something interesting about this book is that it is assembled of many characters from Clamp's other books. There are some characters from every series they've written so far. One rather fun example of this is how every time the group visits a new world, in the background you can see Miyuki, from Clamp's Miyuki-chan in Wonderland, running around in her sailor uniform (although the style varies slightly to match the world they're in). Also, Clamp's xxxHolic is the exact same events as Tsubasa, but from a different perspective. These are more about Yuko and her assistant, Watanuki. I have not read this series yet, but I hope to some time.
In general, I am really liking Tsubasa, and I think that it is a great series of books, that is, if you are okay with reading it in the traditional Japanese style: right to left.