www.libreriaeditricevaticana.com. Vatican publishing house and the source for Roman chant books such as the Graduale Simplex.
www.giamusic.com. G.I.A. Publications sells many of the chant books listed on this webpage, both Solesmes and Roman books.
www.paracletepress.com Sells a wide selection of chant books.
www.ocp.org. O.C.P. Publications sells many of the chant books listed on this webpage, both Solesmes and Roman books.
www.canticanova.com. CanticaNOVA Publications is a publishing house for liturgical music including Gregorian chant; online resource for liturgical and musical planning.
www.musicasacra.com. Website of the Church Music Association of America, with many useful chant resources, such as the communio Psalm verses of the 1974 Graduale Romanum fully notated, and several preconciliar chant books in pdf files, such as the 1912 Antiphonale Romanum and the 1908 Graduale Romanum .
comp.uark.edu/~rlee/otherchant.html, with many links to other helpful sites.
www.vatican.va/archive/bible/, the Bible in English, Italian, Latin, and Spanish from the Vatican . The Latin Bible is the postconciliar Nova Vulgata ("new Vulgate").
www.fourmilab.ch/etexts/www/Vulgate/, the pre-Vatican II Vulgate Bible.
www.archives.nd.edu/cgi-bin/words.exe. "Words" by William Whitaker - see under "Translation Helps" on this website.
Associazione Internazionale Studi di Canto Gregoriano ("international association for the study of Gregorian chant"): www.aiscgre.it/home2.html - Italian-language section, www.aiscgre.de - German-language section, www.cantogregoriano.es - Spanish-language section.
www.gregor-und-taube.de/html/materialien.htm#I.1. Helpfully provides chants of the Graduale Romanum with melodic correction done by German-language scholars. The Second Vatican Council called for the melodic correction of the gradual, but this work has not yet been completed. One can expect a revised gradual to appear in the future, probably still some decades away. In the meantime, this site shows how interesting and compelling the original melodies are. One can probably find ones way around even though the site is in German, since the liturgical year is of course the same.
www.omifacsimiles.com, for purchase of facsimiles of the principal chant manuscripts.
www.cesg.unifr.ch/de/index.htm, Codices Electronici Sangallenses ("Electronic Codices of St. Gall"). Allows one to view many of the important chant manuscripts of St. Gall, e.g. 390/390, Hartker's Antiphonary, which gives early lineless neumes for many of the easier chants in the Graduale Simplex.