A large amount of information is available on the web, including images of manuscripts, scans of old books, freely usable music editions, and more. Here is a small selection.
Ms239 on this site contains one of the early musical notations which have been copied into the Graduale Triplex - the one in black above the stave.
High-resolution images of manuscripts, from St. Gallen and other Swiss libraries. Some of the most important manuscripts with early musical notation are available on this site, including the "Harker Antiphonary" (Cod. Sang. 390/391), the "Cantatorium" (Cod. Sang. 359), and a Calendar, Gradual, Sacramentary (Cod. Sang. 339), probably also by Hartker.
Another important manuscript, the Gradual of St Emmeran of Ratisbon (Regensburg), is available here. Follow "Digitale Angebote", then the "Zur Recherche" link, and search the resulting page for the manuscript reference "Lit.6"
Online editions of the Propers of the Mass, with restored melodies derived from recent research in Beiträge zur Gregorianik. St. Gall neumes are transcribed above the music.
St. Antoine Daniel Kyriale - online edition of the Mass ordinaries.
St. René Goupil Gradual - online edition of the Mass Propers. These are the "standard" versions of the melodies, and the site also has recordings of them.
Online editions of some chants, and other useful resources.
A nice essay.