BibTeX:
@incollection{AbdulBah1886,
  author = {'Abdu'l-Bahá},
  title = {Nine Months at Máh-Kú: The Báb in the Mountain Castle},
  booktitle = {A Traveler's Narrative},
  year = {1886},
  publisher = {Cambridge University Press},
  url = {https://www.gutenberg.org/ebooks/19300},
}

APA:
'Abdu'l-Bahá. (1886). Nine Months at Máh-Kú: The Báb in the Mountain Castle. In *A Traveler's Narrative*. Cambridge University Press.

MLA:
'Abdu'l-Bahá. "Nine Months at Máh-Kú: The Báb in the Mountain Castle." *A Traveler's Narrative*, Cambridge University Press, 1886.

Chicago:
'Abdu'l-Bahá. "Nine Months at Máh-Kú: The Báb in the Mountain Castle." In *A Traveler's Narrative*, Cambridge University Press, 1886.