Ramban on Tefillin
The Ramban (1194-1270) wrote a fabulous book on Jewish thought. I think it's relatively obscure, but it's one of my all-time favorites. Referred to as "Derashah" or "Drashas HaRamban," the book was originally just that: an extended sermon given before the King of Castile.
This work is written in a style similar to Moreh Nevuchim...