Doktor Hoffmann

For those of you who sprechen sie Deutsch, Dr. David Hoffman's (הרה"ג מהר"ר דוד צבי הופמן זצ"ל) Das Buch Leviticus is entirely available on Google Books. (Also Der Schulchan-aruch und die Rabbinen über das Verhältniss der Juden zu Andersgläubigen, if Gothic script is your speed, and some of his commentaries in Hebrew).

A wonderful biographical sketch by Alexander Marx, his beloved talmid and son-in-law, in the latter's "Essays in Jewish Biography."