BibTeX:
@incollection{theBb1976,
  author = {the Báb},
  title = {Letters from the Mountain},
  booktitle = {Selections from the Writings of the Báb},
  year = {1976},
}

APA:
the Báb. (1976). Letters from the Mountain. In *Selections from the Writings of the Báb*.

MLA:
the Báb. "Letters from the Mountain." *Selections from the Writings of the Báb*, 1976.

Chicago:
the Báb. "Letters from the Mountain." In *Selections from the Writings of the Báb*, 1976.