When exploring a subject it makes sense to start with reference works of proven quality.
Unfortunately we have nowadays in education to deal with an attitude problem of modern society: Everything has to be quick, short, easy, not too difficult, not too critical. This has an unavoidable influence on (studying) behaviour and learning outcomes. "Snippet clicking" (clicking form pillar to post without goal or guiding question) and copy-pasting are everywhere.
To become really proficient in something takes, time, energy and commitment.
The accessibility of digital reference works depends on the university where you work or study, or subscriptions you have.
Therefore I don't give direct links here.
General reference works
- Oxford Music Online: formerly GROVE Dictionary of Music and Musicians
- Oxford Art Online
- MGG: Musik in Geschichte und Gegenwart
- Oxford History of Western Music
- Cambridge Histories Online (several volumes on music and drama).
- Oxford Reference Online
- Encyclopaedia Britannica
Manuals
To search (and find) literature and data: use websites of universities. Key words education & research.