BibTeX:
@incollection{JEEsslemont1923,
  author = {J. E. Esslemont},
  title = {In the Heart of Islam: The Báb Proclaims His Mission at Mecca},
  booktitle = {Bahá'u'lláh and the New Era},
  year = {1923},
  publisher = {Bahá'í Publishing Trust},
  url = {https://www.gutenberg.org/cache/epub/19241/pg19241-images.html},
}

APA:
J. E. Esslemont. (1923). In the Heart of Islam: The Báb Proclaims His Mission at Mecca. In *Bahá'u'lláh and the New Era*. Bahá'í Publishing Trust.

MLA:
J. E. Esslemont. "In the Heart of Islam: The Báb Proclaims His Mission at Mecca." *Bahá'u'lláh and the New Era*, Bahá'í Publishing Trust, 1923.

Chicago:
J. E. Esslemont. "In the Heart of Islam: The Báb Proclaims His Mission at Mecca." In *Bahá'u'lláh and the New Era*, Bahá'í Publishing Trust, 1923.