MGG is the premier German music encyclopedia, and includes an extensive article on Beethoven. Google translate is available via the tiny globe icon.
Grove is the classic music encyclopedia, and includes an entry on Beethoven written by numerous Beethoven scholars.
How do I connect from off campus?
Stanford Libraries provides online access to books about Beethoven's life and works. The link above lists entries with newest publications first. Note how the results are refined by access (online) and language (English)