This is THE place to start! But don't limit yourself to Grove only. Online and print.

Online and Print. This is the place to start for German-specific topics. This reference encyclopedia should complement the Grove nicely. For example, the article on J.S. Bach is over 100 pages long.

This is in 2 parts: 1. Men and their works. 2. Techniques, forms, and instruments. This is ideal to complement Grove on French topics. It will also include more obscure French people/topics not covered elsewhere.

This dictionary of music and musicians is divided in 3 parts: terms, biography, and titles & characters. The last part is possibly the most interesting. For example, it provides excellent plots for operas, description of characters in these, different song settings for specific texts, etc. The terms section is probably the best source to get definition of Italian terms.

Comprehensive online encyclopedia devoted to world music research. Contains the full text of the 10-volume print encyclopedia. Includes maps, music examples, bibliographies, discographies, and videographies. A CD sampler accompanies the first 9 vols.

Looking for something a little more succinct? This has short articles on composers, performers, terms, titles (e.g., of operas), etc. The J.S. Bach article, for instance, has 3 pages. Good as a quick reference for any level of readership.

Dutch equivalent of the Grove. Many nice images.

The Swedish and Norwegian equivalent of the Grove. Lots of illustrations. The place to go for any Nordic composer.

Not as general as other dictionaries listed here. Covers mostly people, but also places, instruments, etc. The best resource for these regions.