This is the first place you should search when doing music research. Use it to find background information about your topic, and pay particular attention to the bibliographies at the end of each entry. They can provide you with great sources!
Schubert AND "piano concerto" AND (analysis OR criticism OR interpretation)
Schubert AND (society OR culture OR politics OR philosophy)
Use these databases to find articles on your topic as well as music recordings in the classical music library.