Mercury!!
The Messenger mission was launched in 2004 and will enter orbit around Mercury in 2011.
The Mercury pages at LANL page, the SEDS page, and the NSSDC page.
Nearly everything known about Mercury comes from the flight of a single spacecraft, Mariner 10. It is described in the page from JPL, and another from NSSDC. This list of on-line books has two on Mercury.