Official NASA site gives an overview of the mission Giotto that was designed to study Comet P/Halley. Also gives details of its study of Comet P/Grigg-Skjellerup , which were completed during its extended mission.
Wikipedia: Comet Halley
User-created history of 1P/Halley, the comet better known as Halley's Comet. Includes photos, history, and other information about the comet and its discoverer, Edmond Halley.
Information about the comet, spacecraft that have visited it, and its composition.
ASP: The Return of Halley's Comet
Includes information on Comet Halley and its history, concentrating on its 1986 appearance.