The Karekano manga is drawn by Masami Tsuda. It is running in a monthly manga magazine called LaLa. This shoujo comic began it's run in December 1995. The tankoubon are published by Hakusen Sha. Those of you who have only seen the anime will still be very familiar with the style of the manga, since Anno uses images from the original comic a lot throughout the show.

Karekano is also available translated into Chinese. The covers look very similar to the Japanese books. Translation and publishing is being done by "comics world"

To my knowledge there are currently 11 Japanese tankoubon released, with more on the way. Information reguarding volumes 1-12 is shown below.
Thank you to Venkarel for the ISBNs for volumes 7 and 8, and Souichirou Arima for scanning volume 8. ^_^