-Using dictionaries and encyclopedias
-Selected Dictionaries and Encyclopedias
When using an encyclopedia make sure to check the index first and try to search using varied search terms.
Ex: To understand more about Bach's life when he composed the Brandenburg Concertos, look up the concertos, and separately look up Bach's biographical information.
Think wikipedia, but with more credibility. While more recent encyclopedias may serve you well, don't forget to look at some of the oldest ones as well. Older encyclopedias and dictionaries can occasionally focus on different aspects of music than newer ones and may also mention some of the more obscure parts of music that aren't as frequently studied in the present.