Grove Online (from Oxford Music) is an encyclopedia, so you need to search using general terms and keywords.
Search term examples:
Composer biographies, books about historical eras (such as the baroque, renaissance, etc), and essay collections on a specific topic (such as queerness in music)--all these items can help you gain background information about a topic.
Books can also help you contextualize your topic; you can learn about the settings and environments that composers, theorists, and other citizens worked within.
IUCAT offers print books and e-books results