Hovoth haLevavoth or Kitab ul-Hidayah ila Fara'idh il-Qulub - this book was written in Hebrew letters but the language was Arabic (Rabbeinu Bahiyah went against tradition and used Arabic script for his modern edition) - that makes it so unique. I personally find very fascinating parallel Hebrew-Arabic edition. Few years ago I heard in a sh'iur that the Qafih translation from the original Arabic is far more accurate than the one of Rabbi Yehudah Tibbon (just saying).