One of my favorite manga of all time, Nazo no Kanojo X(Mysterious Girlfriend X in English), by Ueshiba Riichi, is about drool. Yep. Drool.
Akira Tsubaki, in a daze of passion, happens to taste the drool of Mikoto Urabe, asleep on her desk in school. This leads to a mysterious "illness" that can only be temporarily offset by more of her drool.
That semi-paranormal element aside, the story follows their simple and developing relationship. NnKX separates itself from similar such series by focusing on the relationship itself; cutting out excessive drama, and keeping to a small cast of characters including Tsubaki's sister, and Ueno and Oka, friends of Tsubaki and Urabe, respectively, who are also in a relationship. The bond between the characters is noted for being realistic in both its nature and its progression, and slightly Natusume Ono-ish in style.
The series began in 2004 and is published monthly; a one-shot which inspired the full series had been published several months before the series began.