Grove (Oxford) Music Online
Grove Music Online is very useful for getting an introduction to your topic. You can search by types of music, e.g. opera, symphony; by composer; by musical instruments; by topics, such as sound studies; by musical forms, such as sonata, fugue, etc. For most articles, you will find a bibliography. The bibliography may not list the most up-to-date sources, but it is usually a good place to start. If you are researching a composer, a works list will be included. This will provide information about editions of the composer's works, when they were composed, instrumentation, and, if applicable, the volume of a collected works edition where each piece can be located.