The New Grove Dictionary of Music and Musicians has been an invaluable resource for guitar enthusiasts for many years. In addition to the German-language Die Musik aufliert und Gegenwarts, it’s one of the biggest reference works on music and the history of music. I have often used it as a reference when trying to understand music theory. I highly recommend it.
Online, you’ll find many other music dictionaries that cover musical terms, including those written in many languages other than English such as Italian and Spanish. Most online music dictionaries are available free. Some, however, may require a small fee such as the New Grove music dictionary, which offers a wide variety of musical terms along with biographical information about famous guitarists.
Many professional music dictionaries and online music references are also available. The best ones I’ve found are those that allow you to search by genre, term or era. I highly recommend the online edition of Der Zweck und der Musik (the German/English Dictionary). You’ll find it online at: , metal, New Age, piano, rhythm and strumming, pop and other music genres. Just a note of caution: some of these music dictionaries have pages of terms just like the ones I’ve described above. Others aren’t quite as comprehensive. But all music dictionaries have valuable information and an easy to use search engine that will make finding and comparing music terms a breeze.